以下代码在DELPHI 7上调试通过，主要使用了api函数中MultiByteToWidechar

function UnicodeEncode(Str:string;CodePage:integer):WideString;
var
Len:integer;
begin
Len:=Length(Str)+1;
SetLength(Result,Len);
Len:=MultiByteToWideChar(CodePage,0,PChar(Str),-1,PWideChar(Result),Len);
SetLength(Result,Len-1); //end is #0
end;

function UnicodeDecode(Str:WideString;CodePage:integer):string;
var
Len:integer;
begin
Len:=Length(Str)*2+1; //one for #0
SetLength(Result,Len);
Len:=WideCharToMultiByte(CodePage,0,PWideChar(Str),-1,PChar(Result),Len,nil,nil);
SetLength(Result,Len-1);
end;

function Gb2Big5(Str:string):string;
begin
SetLength(Result,Length(Str));
LCMapString(GetUserDefaultLCID,LCMAP_TRADITIONAL_CHINESE,
PChar(Str),Length(Str),
PChar(Result),Length(Result));
Result:=UnicodeDecode(UnicodeEncode(Result,936),950);
end;

function Big52Gb(Str:string):string;
begin
Str:=UnicodeDecode(UnicodeEncode(Str,950),936);
SetLength(Result,Length(Str));
LCMapString(GetUserDefaultLCID,LCMAP_SIMPLIFIED_CHINESE,
PChar(Str),Length(Str),
PChar(Result),Length(Result));
end;

关键使用了UnicodeToUtf8这个函数
function Utf8Encode(const WS: WideString): UTF8String;
var
L: Integer;
Temp: UTF8String;
begin
Result := '';
if WS = '' then Exit;
SetLength(Temp, Length(WS) * 3); // SetLength includes space for null terminator
L := UnicodeToUtf8(PChar(Temp), Length(Temp)+1, PWideChar(WS), Length(WS));
if L > 0 then
SetLength(Temp, L-1)
else
Temp := '';
Result := Temp;
end;