The wide use of galvanized steel in automobile manufacturing brings much challenge to the laser welding process of roof to body-side. The fillet joint is the most effective way to solve the problem of welding pores in the process of laser welding. However,there is little research on this complicated joint process. Focused on this problem,taking metallographic size of welding seams as weld quality criteria,response surface methodology (RSM) is used to study the influence of laser beam on welding seam quality. Key welding process parameters and the optimum welding parameters are concluded.