Abstract: Based on the theory of fluid dynamics, thermal mechanical coupled model of weld pool, keyhole and flow field of K-PAW (Keyhole Plasma Arc Welding) are studied with FLUENT software in this paper. A optimized model of energy and arc pressure changed quadratically as the depth of keyhole increasing is proposed. Furthermore, the forces maintaining keyhole stable act inside of the keyhole and on the physical boundary at the same time. The numerical simulation of penetration process and welding process after penetration are realized initially with this method. It takes 2.15 s to penetrate the whole workpiece at the calculated result which is in agreement with the penetrating time of experimental measurement. And the forms of keyhole agree well during dynamic penetrating process. The trend of fusion line is same basically.