diff --git a/src/index.js b/src/index.js
index d9848b5..f02f863 100644
--- a/src/index.js
+++ b/src/index.js
@@ -27,7 +27,7 @@ export default class Alioss {
 
   // 授权签名, 用于临时下载私有bucket的文件
   auth(key) {
-    var time = Math.floor(Date.now() / 1000) + 1800 // 半小时内
+    let time = Math.floor(Date.now() / 1000) + 1800 // 半小时内
 
     return hmac(
       'SHA-1',
@@ -47,14 +47,14 @@ export default class Alioss {
    * size: 大小限制, 单位 MB  默认10MB
    */
   sign(dir = '', size = 10) {
-    var time = new Date()
-    var params = {
+    let time = new Date()
+    let params = {
       conditions: [
         ['content-length-range', 0, Math.floor(1024 * 1024 * size)],
         ['starts-with', '$key', dir ? dir.replace(/\/+$/, '') + '/' : '']
       ]
     }
-    var policy = ''
+    let policy = ''
 
     time.setTime(time.getTime() + 60 * 60 * 1000) // 60分钟内有效
     params.expiration = time.toISOString()
@@ -67,8 +67,8 @@ export default class Alioss {
   }
 
   list({ prefix = '', delimiter = '/', max = 1000, token } = {}) {
-    var time = new Date().toGMTString()
-    var query = {
+    let time = new Date().toGMTString()
+    let query = {
       'list-type': 2,
       prefix,
       delimiter,
@@ -103,7 +103,7 @@ export default class Alioss {
    * key: 要保存的文件名, 带完整路径
    */
   upload(auth, file, key) {
-    var body = new FormData()
+    let body = new FormData()
 
     if (!file.type) {
       let ext = file.name.split('.').pop()
@@ -124,8 +124,8 @@ export default class Alioss {
   }
 
   copy(origin, target) {
-    var time = new Date().toGMTString()
-    var headers = {
+    let time = new Date().toGMTString()
+    let headers = {
       'x-oss-date': time,
       'x-oss-copy-source': `/${this.#bucket}/${encodeURIComponent(origin)}`
     }
@@ -153,7 +153,7 @@ export default class Alioss {
   }
 
   delete(key) {
-    var time = new Date().toGMTString()
+    let time = new Date().toGMTString()
 
     return hmac(
       'SHA-1',
diff --git a/src/lib/helper.js b/src/lib/helper.js
index ddfea92..dba80d2 100644
--- a/src/lib/helper.js
+++ b/src/lib/helper.js
@@ -49,12 +49,12 @@ export function fixFile(name, data) {
 export function str2sign(
   method = 'GET',
   bucket,
-  { time, headers = {}, key, query } = {}
+  { time, contentType = '', headers = {}, key, query } = {}
 ) {
   let arr = [
     method,
     '', // 请求内容的md5值, 用于服务端校验文件是否完整. 可以为空
-    '', // 请求文件的content-type类型, 可以为空
+    contentType, // 请求文件的content-type类型, 可以为空。不为空时, 注意表单提交时要统一
     time,
     Object.keys(headers)
       .sort()