string fileName = "name.apk";//客户端保存的文件名
        string filePath = Server.MapPath("apk/KMYY.apk");//路径 //以字符流的形式下载文件
        FileStream fs = new FileStream(filePath, FileMode.Open);
        byte[] bytes = new byte[(int)fs.Length]; fs.Read(bytes, 0, bytes.Length);
        fs.Close();

             Response.ContentType = "application/octet-stream"; //通知浏览器下载文件而不是打开
       //Response.ContentType = "application/vnd.android.package-archive";
        Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));
        Response.BinaryWrite(bytes);
        Response.Flush();
        Response.End();

C#下载apk文件