Plasma is produced in deep penetration laser welding. It influencs the interaction between laser source and workpiece plate according to laser beam absorption, reflection as well as refraction etc. Based on micro-focus high speed photograph, dynamic behavior and size of plasma were studied in different laser power and different flow rate of side assist gas. It is discovered that the plasma is larger and more unstable with the increase of laser power density. And the whole process is more susceptible to LSC(Laser Supported Combustion), even resulting in welding discontinuity. By increasing the flow rate of side-assist gas, LSC is supressed and the size of the plasma becomes smaller. Correspondingly, the penetration is slightly deeper with smaller plasma.