Partial penetration laser welding experiment of DC01 galvanized steel plate and SUS304 austenite stainless steel as well as pre-deformation experiment are conducted by fiber laser. The deflection and deformation profile of SUS304 steel plate (lower plate) are measured and the mechanism of the micro bulging distortion of SUS304 steel plate surface after partial penetration laser welding. The results show that angular distortion is an important reason that lead to the micro bulging distortion of austenite stainless steel surface. Pre-deformation can decrease the maximum deformation height significantly. However, the plastic bulging distortion still occurs in micro bulging distortion area. It is concluded that the main reason of micro bulging distortion of SUS304 steel plate at weldless side in partial penetration laser welding is the combination of angular distortion and plastic deformation caused by thermal expansion during welding.