public void DownLoad( )
{
     string filePath = Server.MapPath( @"\UserFile\" );//这里注意了,你得指明要下载文件的路径.

     if ( System.IO.File.Exists( filePath ) )
     {
          FileInfo file = new FileInfo( filePath );
          Response.ContentEncoding = System.Text.Encoding.GetEncoding( "UTF-8" ); //解决中文乱码
      Response.AddHeader( "Content-Disposition", "attachment; filename=" + Server.UrlEncode( file.Name ) ); //解决中文文件名乱码    
      Response.AddHeader( "Content-length", file.Length.ToString() );
          Response.ContentType = "appliction/octet-stream";
          Response.WriteFile( file.FullName );
          Response.End();
          }
}

asp.net下载的方法