查詢網上的方法發現有以下幾種：

1.workbook.Write(ms)生成的 MemoryStream後，使用了 ms.GetBuffer()返回檔案內容，導致生成的 Excel檔案結尾處有大量的 00（空位元組），改為 ms.ToArray()即可得到正常的檔案了。

2.編碼不一致。可以修改如下：

 
                Response.ContentType = "application/vnd.openxmlformats - officedocument.spreadsheetml.sheet";
                Response.AppendHeader("Content-Type", "text/html; charset=GB2312");
                Response.AddHeader("Content-Disposition", string.Format("attachment; filename={0}.xlsx", fileName));
                Response.Charset = "GB2312";
                Response.ContentEncoding = Encoding.GetEncoding("GB2312");
 

但是這兩種都不是我的問題，最後發現，是流沒有正常關閉。最後加了一句Response.Close(); 問題解決。

具體程式碼示例如下：

        ///
        /// 匯出檔案到瀏覽器
        ///
        ///
        ///
        ///檔名稱
        private void ExportExcel(XSSFWorkbook workbook, HttpResponse Response, string fileName)
        {
            using (var exportData = new MemoryStream())
            {
                workbook.Write(exportData);
                Response.Buffer = true;
                Response.Clear();
                Response.ClearHeaders();
                Response.ClearContent();
                //response.ContentType = "application/ms-excel";
                Response.ContentType = "application/vnd.openxmlformats - officedocument.spreadsheetml.sheet";
                Response.AppendHeader("Content-Type", "text/html; charset=GB2312");
                // Response.AddHeader("Content-Length", exportData.Length.ToString());
                Response.AddHeader("Content-Disposition", string.Format("attachment; filename={0}.xlsx", fileName));
                Response.Charset = "GB2312";
                Response.ContentEncoding = Encoding.GetEncoding("GB2312");
                Response.BinaryWrite(exportData.ToArray());// ms.ToArray()   GetBuffer
                Response.Flush();
                Response.Close();
            }
        }