'ANSI　　　　　　　　 无格式定义；
'Unicode 　　　　　　 前两个字节为FFFE；
'Unicode big endian　 前两字节为FEFF；
'UTF-8　 　　　　　　 前两字节为EFBB；
Function checkFileEncoder(FilePath)
 Dim stream, binheader
 Set stream = Server.CreateObject("adodb.stream")
 stream.Type = 1'设置为2进制，主要是为了判断文件编码
 stream.Mode = 3 'forreadwrite
 stream.Open
 stream.LoadFromFile FilePath
 stream.position = 0
 binheader = stream.read(2)
 stream.Close
 Set stream = Nothing
 If LenB(binheader) = 1 Then checkFileEncoder = "gb2312"
 If AscB(MidB(binheader, 1, 1)) = &HEF And AscB(MidB(binheader, 2, 1)) = &HBB Then
   checkFileEncoder = "utf-8"
 ElseIf AscB(MidB(binheader, 1, 1)) = &HFF And AscB(MidB(binheader, 2, 1)) = &HFE Then
  checkFileEncoder = "unicode"
 Else
  checkFileEncoder = "gb2312"
 End If
End Function

Sub Read(FilePath)
  Dim stream, fileEncoder
  fileEncoder = checkFileEncoder(FilePath) '获取文件编码格式
  Set stream = Server.CreateObject("adodb.stream")
  stream.Charset = fileEncoder '设置编码格式
  stream.Type = 2 
  stream.Mode = 3 
  stream.Open
  stream.LoadFromFile FilePath
  MyResponse.Write  stream.ReadText() 
  stream.Close
  Set stream = Nothing
End Sub

call Read(Server.MapPath("data.txt"))