解决了
现在贴出代码

  public static void ExportToDownLoad(byte[] data, string filename)
        {
            GSPCore.Common.CommonBLL.GetContextType(System.IO.Path.GetExtension(filename)); 
            System.Web.HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment; filename=" 
             + System.Web.HttpUtility.UrlEncode(filename, System.Text.Encoding.UTF8)); 
            //这里就是 读取什么类型 
             //文件返回什么响应头
            System.Web.HttpContext.Current.Response.BinaryWrite(data);
            System.Web.HttpContext.Current.Response.Flush();
            System.Web.HttpContext.Current.Response.End();
        }

这样子的话,调用需要改动

  using (MemoryStream ms = new MemoryStream())
   { 
      xlsBK.Save(ms, FileFormatType.Excel2007Xlsx);//默认支持xls版，需要修改指定版本-->就是这句话
       ExportToDownLoad(ms.ToArray(), exlname);
       xlsBK = null;  
   }