Abstract: Laser induced plasma is an important physical phenomenon in laser deep penetration welding. It has stronger relationsip to stability of process, quality of weld, and efficiency of laser energy. In this paper, two methods were used to study the dynamic behavior of plasma and the influences on stability of welding process. These methods were high-speed camera and optical signal monitoring. The results showed that the dynamic process of plasma can be divided into four steps, (1) material vaporing; (2) plasma increasing; (3) plasma bombing and separating; (4) plasma scattering. The main reason affected welding process stability is the fluctuation of plasma between non-penetration and penetration process.