Abstract: In this paper,a mathematical model of keyhole mechanism of laser deep welding has been analyzed.We studied the relationship of the stability of weld pool with metallic vapor pressure and the relationship of metallic vapor with plasma.The effect of laser weld parameters(including laser mode,laser power,focusing condition,welding speed and assistant gas etc.)on weld pool behavior have been investigated in detail.Finally,we also have analyzed the weld pool behavior related weld structure and defects(such as crack and porosity etc.).