System.IO.Stream iStream = null;
     byte[] buffer = new Byte[10240];
     int length;
     long dataToRead;
     string filepath = "文件存放地址";
     string filename = System.IO.Path.GetFileName(filepath);
     iStream = new System.IO.FileStream(filepath, System.IO.FileMode.Open, System.IO.FileAccess.Read,System.IO.FileShare.Read);
     Response.Clear();
     dataToRead = iStream.Length;
     long p = 0;
     if(Request.Headers["Range"]!=null)
     {
      Response.StatusCode = 206;
      p = long.Parse( Request.Headers["Range"].Replace("bytes=","").Replace("-",""));
     }
     if(p != 0)
     {
      Response.AddHeader("Content-Range","bytes " + p.ToString() + "-" + ((long)(dataToRead - 1)).ToString() + "/" + dataToRead.ToString()); 
     }
     Response.AddHeader("Content-Length",((long)(dataToRead-p)).ToString());
     Response.ContentType = "application/octet-stream";
     Response.AddHeader("Content-Disposition", "attachment; filename=" + System.Web.HttpUtility.UrlEncode("真实文件名"));
     iStream.Position = p;
     dataToRead = dataToRead - p;
     while (dataToRead > 0)
     {
      if (Response.IsClientConnected) 
      {
       length = iStream.Read(buffer, 0, 10240);
       Response.OutputStream.Write(buffer, 0, length);
       Response.Flush();
       buffer= new Byte[10240];
       dataToRead = dataToRead - length;
      }
      else
      {
       dataToRead = -1;
      }
     }
     iStream.Close();
     Response.End();