The tensile stress and microstructure distribution of 316 L stainless steel welding joint are heterogeneous, so it introduces more risks of stress damage corrosion. The welded joint was treated by different power density (Laser Shock Peening (LSP) to overcome this problem). Through residual stress test, XRD and SEM, the residual stress distribution, phase structure and microstructure were characterized, and the mechanism of Laser Shock Peening (LSP) was discussed. The results showed that by the use of different power density, the residual tensile stress of welded joint decreased in comparison with welded sample .When the power density increased to 5.52 GW/cm2 or 6.63 GW/cm2, these residual tensile stresses generated were superseded by the compressive residual stresses, and the maximum compressive residual stress reached about -100MPa. With the increasing of laser power density, the severe plastic deformation appeared in the surface layer of welded joint, the crystal grain was stretched, the affected layer became deeper. These are useful for the improvement of the corrosion resistance of welded joint. But phase transformation happened when the power density increased to 6.63 GW/cm2.It is harmful to the performance of stress corrosion. Therefore, a specific laser power density turns out to be an effective approach to improve the stress corrosion resistance.