using (FileStream fsteam = File.OpenRead(@"D:\tmp\1.txt"))
            {
                byte[] buff = new byte[fsteam.Length];
                fsteam.Read(buff, 0, buff.Length);
                string str = Encoding.GetEncoding("GB18030").GetString(buff);
                //string str = Encoding.GetEncoding("gb2312").GetString(buff);
                Console.WriteLine(str);
            }

------解决方案--------------------

StreamReader sr  = new StreamReader("x:\\xx.txt",Encoding.GetEncoding("GB18030"));
string content = sr.ReadToEnd();