procedure TMainFrm.ScreenShot(x : integer; y : integer; Width : integer; Height : integer; bm : TBitMap);
var
dc: HDC; lpPal : PLOGPALETTE;
begin
  // 检测所需抓屏的区域
  if ((Width = 0) or (Height = 0)) then exit;
  bm.Width := Width;
  bm.Height := Height;
  //获取设备上下文
  dc := GetDc(0);
  if (dc = 0) then exit;
  {do we have a palette device?}
  if (GetDeviceCaps(dc, RASTERCAPS) AND
    RC_PALETTE = RC_PALETTE) then
    begin
    {allocate memory for a logical palette}
    GetMem(lpPal, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)));
    {zero it out to be neat}
    FillChar(lpPal^, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)), #0);
    {fill in the palette version}
    lpPal^.palVersion := $300;
    {grab the system palette entries}
    lpPal^.palNumEntries :=
      GetSystemPaletteEntries(dc,0,256,lpPal^.palPalEntry);
    if (lpPal^.PalNumEntries <> 0) then
    begin
      {create the palette}
      bm.Palette := CreatePalette(lpPal^);
    end;
    FreeMem(lpPal, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)));
    end;
  {copy from the screen to the bitmap}
  BitBlt(bm.Canvas.Handle,0,0,Width,Height,Dc,x,y,SRCCOPY);
  {release the screen dc}
  ReleaseDc(0, dc);
end;

//调用方法
procedure TMainFrm.Button1Click(Sender: TObject);
var
  tBM : TBitmap;
begin
  try
    tBM := TBitmap.Create;
    ScreenShot(0,0,Screen.Width,Screen.height,tBM);
    tBM.SaveToFile('c:\ScreenShot.BMP');
  finally
    tBm.FreeImage;
    FreeAndNil(tBM);
  end;
end;