Code
 public class AutoDownloadHandler : IHttpHandler
    {
        #region IHttpHandler Members

        public bool IsReusable
        {
            get { return false; }
        }

        public void ProcessRequest(HttpContext context)
        {
            string fileName = “地址栏传入的文件名参数”
            string siteUrl = System.Configuration.ConfigurationManager.AppSettings["FtpUrl"].ToString();
            string user = System.Configuration.ConfigurationManager.AppSettings["FtpUser"].ToString();
            string passWord = System.Configuration.ConfigurationManager.AppSettings["FtpPassWord"].ToString();

            try
            {
                string filePath = siteUrl + fileName;

                HttpWebRequest httpWebRequest;
                httpWebRequest = (HttpWebRequest)HttpWebRequest.Create(filePath);
                NetworkCredential credential= new NetworkCredential(user, passWord);
                httpWebRequest.Credentials = credential;
                HttpWebResponse httpWebResponse;
                httpWebResponse = (HttpWebResponse)httpWebRequest.GetResponse();
                
                Stream responseStream = httpWebResponse.GetResponseStream();
 
                MemoryStream stmMemory = new MemoryStream();
                byte[] buffer = new byte[64 * 1024];
                int i;
                while ((i = responseStream.Read(buffer, 0, buffer.Length)) > 0)
                {
                    stmMemory.Write(buffer, 0, i);
                }
                byte[] arrayByte = stmMemory.ToArray();
                stmMemory.Close();
                responseStream.Close();

                HttpContext.Current.Response.Buffer = true;
                HttpContext.Current.Response.Clear();
                HttpContext.Current.Response.ContentType = GetMimeType(fileName);
                HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));
                HttpContext.Current.Response.AppendHeader("Content-Length", arrayByte.Length.ToString());
                HttpContext.Current.Response.BinaryWrite(arrayByte);
                HttpContext.Current.Response.Flush();
                HttpContext.Current.Response.End();

            }
            catch (Exception ex)
            {
               
            }
        }

        #endregion
    }