procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  ReleaseCapture;
  SendMessage(Handle,WM_NCLBUTTONDOWN,HTCAPTION,0);
end;

  public
    { public declarations }
    posX, posY: Integer;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  posX := Mouse.CursorPos.X - Form1.Left;
  posY := Mouse.CursorPos.Y - Form1.Top;
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState;
  X, Y: Integer);
begin
  if Shift =[ssLeft] then
  begin
    Form1.Left := Mouse.CursorPos.X - posX; // 新的位置
    Form1.Top := Mouse.CursorPos.Y - posY;
  end;
end;