Laser shock processing (LSP) is an innovative approach, which effectively improves the mechanical behavior of metallic structures by introducing compressive residual stress. To evaluate the residual stress evolution in low-carbon 13Cr4Ni martensitic stainless steel multi-pass butt-welded joints induced by LSP, a two-step numerical simulation including welding analysis, at first, followed by LSP calculation with the simulated welding stress results being taken into account, was performed based on ABAQUS software. Effects of LSP parameters such as power density, spot size, overlapping rate and numbers of laser shock on the residual stress variations, were systematically investigated. To validate the reliability and accuracy of the numerical simulation, experiments of welding and LSP were conducted in sequence. The residual stress after welding and LSP were investigated by X-ray diffraction method. Results demonstrate that the simulated results show a good agreement with the experimental datas. The welding residual stress distribution is uneven. Larger tensile stresses appear on the weld surface and its adjacent heat-affected zone, which could be converted into high-level compressive stress after LSP. Furthermore, an ideal residual stress field can be obtained after two successive laser shocks with an overlap rate of 75% when the power density, spot diameter, and pulse width are 7.6 GW/cm2, 4 mm, and 25 ns, respectively.