<form id="uploadForm"> 
   <p>上传文件：<input type="file" name="file" /></p> 
   <input type="button" value="上传" onclick="upload()" /> 
</form>
function upload() { 
   var formData = new FormData($("#uploadForm")[0]); 
   $.ajax({ 
     url: '', 
     type: 'POST', 
     data: formData, 
     async: false, 
     cache: false, 
     contentType: false, 
     processData: false, 
     success: function(data) {
     }, 
     error: function(data) {   
     } 
   }); 
}

需要使用

 async: false