string str = "5Y2a5a6i5Zut55So5oi35biQ5oi35r+A5rS76YKu5Lu2";
            byte[] bytes = Convert.FromBase64String(str);
            Console.WriteLine(Encoding.UTF8.GetString(bytes));

------解决方案--------------------

            //转成utf-8

           string str = "中国科技论文在线-首发论文退稿";

           byte[] utf8 = Encoding.UTF8.GetBytes(str);
           string s3 = "";
           foreach (byte b in utf8)
           {
               s3 += "=" + string.Format("{0:X2}", b);

           }
           Response.Write(s3 + "<br />");




           //转成汉字
           string cd = "=E4=B8=AD=E5=9B=BD=E7=A7=91=E6=8A=80=E8=AE=BA=E6=96=87=E5=9C=A8=E7=BA=BF=2D=E9=A6=96=E5=8F=91=E8=AE=BA=E6=96=87=E9=80=80=E7=A8=BF";
           string[] b6 = cd.Split(new char[] { '=' }, StringSplitOptions.RemoveEmptyEntries);
           byte[] bs = new byte[b6.Length];
           int i = 0;
           while (i<b6.Length)
           {
               bs[i] = (byte)Convert.ToByte(b6[i], 16);

               i++;
           }

           Response.Write(Encoding.GetEncoding("UTF-8").GetString(bs));