Abstract: Considering plasma effect on the surface of the workpiece, a numerical simulation model of keyhole and weld pool coupled with heat and mass transfer was established. The adaptive heat source model was used with the change of shape of keyhole in real time. Mass transfering and energy lossing caused by evaporation was considered in the model, and the momentum sinking due to solidification was dealt with enthalpy-porosity technique. The keyhole wall was calculated by fluid volume equation, mainly considering recoil pressure induced by the metal evaporation, surface tension and hydrostatic pressure. The results demonstrated that, spatters are shaped such as weld flash and reinforcement after keyhole formation and perforation. The upper and lower welding width is slightly larger than the middle induced by raleigh scattering of laser, reflection and expansion of plasma. The calculated values were basically consistent with the experimental results in shape and size of the weld.