Abstract: Of aluminum alloy 5154 A plate were lap welded by Nd: YAG laser with preheating and without preheating,respectively. The results indicate that the preheating play a major role in laser welding of aluminum alloy. With the preheat temperature rising,the depth and width of weld increase. The best preheat temperature range is 200-300 ℃. When the preheat temperature is constant,the weld width reduces with the increase of welding speed. But when the welding speed is constant,the depth and width of weld can increase because the preheating can increase the laser absorption rate and reduce the energy needed for the molten metal.