String xx  = "要写的内容" ;
String filePath = @"D:\xx.txt";
if (System.IO.File.Exists(filePath))
  System.IO.File.WriteAllText(filePath, xx + Environment.NewLine + System.IO.File.ReadAllText(filePath, Encoding.GetEncoding("GB2312")), Encoding.GetEncoding("GB2312"));
else
  System.IO.File.WriteAllText(filePath, xx, Encoding.GetEncoding("GB2312"));