diff --git a/problems/0509.斐波那契数.md b/problems/0509.斐波那契数.md
index 1d17784d..7c899195 100644
--- a/problems/0509.斐波那契数.md
+++ b/problems/0509.斐波那契数.md
@@ -234,6 +234,7 @@ func fib(n int) int {
 }
 ```
 ### Javascript 
+解法一
 ```Javascript
 var fib = function(n) {
     let dp = [0, 1]
@@ -244,6 +245,23 @@ var fib = function(n) {
     return dp[n]
 };
 ```
+解法二：时间复杂度O(N)，空间复杂度O(1)
+```Javascript
+var fib = function(n) {
+    // 动规状态转移中，当前结果只依赖前两个元素的结果，所以只要两个变量代替dp数组记录状态过程。将空间复杂度降到O(1)
+    let pre1 = 1
+    let pre2 = 0
+    let temp
+    if (n === 0) return 0
+    if (n === 1) return 1
+    for(let i = 2; i <= n; i++) {
+        temp = pre1
+        pre1 = pre1 + pre2
+        pre2 = temp
+    }
+    return pre1
+};
+```
 
 TypeScript