diff --git a/crates/oxide_builtins/src/bigint.rs b/crates/oxide_builtins/src/bigint.rs
index 4ff6d6b2..fc9e2f52 100644
--- a/crates/oxide_builtins/src/bigint.rs
+++ b/crates/oxide_builtins/src/bigint.rs
@@ -2,23 +2,17 @@ use num_bigint::BigInt;
 use oxide_runtime_api::{to_primitive, NativeResult, ToPrimitiveHint, VmHost};
 use oxide_types::value::JsValue;
 
-/// JS `BigInt(value)` 构造逻辑：
-/// - `new BigInt()` 抛 TypeError（BigInt 不可 new）
-/// - 无参 → `0n`
-/// - 其余参数先 ToPrimitive(number)，Number 走 NumberToBigInt，其余走 ToBigInt。
+/// JS `BigInt(value)` 构造逻辑（§21.2.1.1）：
+/// - new 形态无条件抛 TypeError（BigInt 不可 new）；
+/// - 无参走 ToBigInt(undefined) → TypeError；
+/// - Number 参数走 NumberToBigInt（非整数抛 RangeError，幅值无 2^53 上界），
+///   其余参数走 ToBigInt。
 pub fn bigint_constructor<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
-    if args.len() > 1 {
-        let this_val = vm.reg(args[0]);
-        if this_val.is_object() {
-            // new 语义：BigInt 不可 new，抛 TypeError。
-            return NativeResult::Err(crate::error::create_type_error(vm, "BigInt is not a constructor"));
-        }
+    if vm.constructing_native() {
+        return NativeResult::Err(crate::error::create_type_error(vm, "BigInt is not a constructor"));
     }
-    // 无参 BigInt() → 0n（注意 BigInt(undefined) 必须抛 TypeError）。
-    if args.len() <= 1 {
-        return NativeResult::Ok(vm.new_bigint(BigInt::from(0)));
-    }
-    let val = vm.reg(args[1]);
+    // 无参 BigInt() 等价于 ToBigInt(undefined) → TypeError，与显式 undefined 同路。
+    let val = if args.len() > 1 { vm.reg(args[1]) } else { JsValue::undefined() };
     // ToPrimitive(value, number)：对象先出盒，用户 @@toPrimitive/valueOf/toString
     // 抛出的异常原样传播，且装箱转换的异常只触发一次。
     let prim = match to_primitive(val, ToPrimitiveHint::Number, vm) {
@@ -42,7 +36,7 @@ pub fn bigint_constructor<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
                 "The number cannot be converted to a BigInt because it is not an integer",
             ));
         }
-        return NativeResult::Ok(vm.new_bigint(BigInt::from(d.trunc() as i128)));
+        return NativeResult::Ok(vm.new_bigint(integer_double_to_bigint(d)));
     }
     // 步骤 4：其它原始类型走 ToBigInt。
     match to_bigint(vm, prim) {
@@ -51,6 +45,29 @@ pub fn bigint_constructor<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
     }
 }
 
+/// NumberToBigInt 步骤 3：整数 double 的精确值 ℤ(x)。
+/// |x| < 2^53 由尾数右移指数位直转；|x| ≥ 2^53 位分解（尾数左移 exp-1075 位），
+/// 全程精确无饱和。-0 返回 0（BigInt 无负零）。
+fn integer_double_to_bigint(d: f64) -> BigInt {
+    let bits = d.to_bits();
+    let negative = bits >> 63 != 0;
+    let exp_field = (bits >> 52) & 0x7FF;
+    let significand = bits & 0xF_FFFF_FFFF_FFFF;
+    if exp_field == 0 {
+        return BigInt::from(0);
+    }
+    let mag = if exp_field < 1075 {
+        BigInt::from((0x10_0000_0000_0000u64 + significand) >> (1075 - exp_field))
+    } else {
+        BigInt::from(0x10_0000_0000_0000u64 + significand) << (exp_field - 1075)
+    };
+    if negative {
+        -mag
+    } else {
+        mag
+    }
+}
+
 /// `BigInt.prototype.toString(radix)`：thisBigIntValue 后按 radix 输出。
 pub fn bigint_to_string<H: VmHost>(vm: &mut H, args: &[u8]) -> NativeResult {
     let this_val = vm.reg(args[0]);
diff --git a/crates/oxide_builtins/tests/bigint_constructor_tests.rs b/crates/oxide_builtins/tests/bigint_constructor_tests.rs
new file mode 100644
index 00000000..24e17895
--- /dev/null
+++ b/crates/oxide_builtins/tests/bigint_constructor_tests.rs
@@ -0,0 +1,91 @@
+use std::sync::Arc;
+
+use oxide_compiler::compiler::Compiler;
+use oxide_types::value::JsValue;
+use oxide_vm::vm::Vm;
+
+fn eval(source: &str) -> Result<JsValue, String> {
+    let allocator = oxide_parser::Allocator::default();
+    let program = oxide_parser::parse(&allocator, source).map_err(|e| format!("Parse error: {:?}", e))?;
+    let module = Compiler::new().compile(&program).map_err(|e| format!("Compile error: {}", e))?;
+    let mut vm = Vm::new();
+    vm.run(&Arc::new(module))
+}
+
+/// 求值一个应得布尔结果的表达式并断言其类型。
+fn eval_bool(source: &str) -> bool {
+    let result = eval(source).expect("eval should succeed");
+    assert!(result.is_bool(), "expected boolean result, got {result:?}");
+    result.as_bool()
+}
+
+// --- Number 参数：NumberToBigInt 精确值 ---
+
+/// 1e300 的 double 精确值（f64 位分解的整数部分），非十进制 10^300。
+const ONE_E300: &str = "1000000000000000052504760255204420248704468581108159154915854115511802457988908195786371375080447864043704443832883878176942523235360430575644792184786706982848387200926575803737830233794788090059368953234970799945081119038967640880074652742780142494579258788820056842838115669472196386865459400540160";
+
+#[test]
+fn double_1e300_exact_value() {
+    assert!(eval_bool(&format!("BigInt(1e300) === {ONE_E300}n")));
+    assert!(eval_bool(&format!("BigInt(-1e300) === -{ONE_E300}n")));
+}
+
+#[test]
+fn double_two_pow_53_exact_value() {
+    assert!(eval_bool("BigInt(9007199254740992) === 9007199254740992n"));
+    assert!(eval_bool("BigInt(9007199254740991.0) === 9007199254740991n"));
+}
+
+#[test]
+fn double_between_two_pow_67_exact_value() {
+    assert!(eval_bool("BigInt(2.5e20) === 250000000000000000000n"));
+    assert!(eval_bool("BigInt(3500000000000000.0) === 3500000000000000n"));
+}
+
+#[test]
+fn double_negative_zero_is_zero_bigint() {
+    assert!(eval_bool("BigInt(-0.0) === 0n"));
+}
+
+// --- 构造形态：new 无条件 TypeError ---
+
+#[test]
+fn new_bigint_no_arg_throws_type_error() {
+    assert!(eval_bool("try { new BigInt(); 0 } catch (e) { e instanceof TypeError }"));
+}
+
+#[test]
+fn new_bigint_with_value_throws_type_error() {
+    assert!(eval_bool("try { new BigInt(1); 0 } catch (e) { e instanceof TypeError }"));
+}
+
+// --- 无参：ToBigInt(undefined) → TypeError ---
+
+#[test]
+fn no_arg_throws_type_error() {
+    assert!(eval_bool("try { BigInt(); 0 } catch (e) { e instanceof TypeError }"));
+}
+
+#[test]
+fn explicit_undefined_throws_type_error() {
+    assert!(eval_bool("try { BigInt(undefined); 0 } catch (e) { e instanceof TypeError }"));
+}
+
+// --- 字符串参数：数字分隔符非法 ---
+
+#[test]
+fn decimal_digit_separator_syntax_error() {
+    assert!(eval_bool("try { BigInt(\"1_0\"); 0 } catch (e) { e instanceof SyntaxError }"));
+}
+
+#[test]
+fn hex_digit_separator_syntax_error() {
+    assert!(eval_bool("try { BigInt(\"0x1_F\"); 0 } catch (e) { e instanceof SyntaxError }"));
+}
+
+#[test]
+fn plain_string_forms_unaffected() {
+    assert!(eval_bool("BigInt(\"10\") === 10n"));
+    assert!(eval_bool("BigInt(\"0x1F\") === 31n"));
+    assert!(eval_bool("BigInt(\"  0o17  \") === 15n"));
+}
diff --git a/crates/oxide_runtime_api/src/coercion.rs b/crates/oxide_runtime_api/src/coercion.rs
index cc13f32e..d9d0044e 100644
--- a/crates/oxide_runtime_api/src/coercion.rs
+++ b/crates/oxide_runtime_api/src/coercion.rs
@@ -960,7 +960,16 @@ fn string_to_bigint_full<H: VmHost>(host: &mut H, s: &str) -> Result<JsValue, St
         (10u32, rest)
     };
     if digits.is_empty() {
-        return Err(host.error_message_text("SyntaxError", "Cannot convert string to BigInt: invalid integer literal"));
+        return Err(
+            host.error_message_text("SyntaxError", "Cannot convert string to a BigInt: invalid integer literal")
+        );
+    }
+    // StringIntegerLiteral 四种基数语法均不含数字分隔符；parse_bytes 会接受
+    // 下划线，故先显式拒绝。
+    if digits.contains('_') {
+        return Err(
+            host.error_message_text("SyntaxError", "Cannot convert string to a BigInt: invalid integer literal")
+        );
     }
     let m = num_bigint::BigInt::parse_bytes(digits.as_bytes(), radix).ok_or_else(|| {
         host.error_message_text("SyntaxError", "Cannot convert string to BigInt: invalid integer literal")