Numerical simulation is an effective way to predict the residual stress field induced by laser shock processing (LSP) and study the effects of laser shock parameters on the residual stress field .The finite element sorftware ANSYS/LS-DYNA was applied to simulate the three-dimensional (3D) residual stress field of 40Cr steel surface layer induced by LSP. The 3D finite element model of laser shock processing 40Cr steel was built. The numerical simulation of residual stress field of 40Cr steel surface layer induced by LSP was realized. The effects of laser shock parameters, intensity, pulse width and beam spot, on the residual stress field of 40Cr steel surface layer were studied by simulation. The result of numerical simulation indicates that the value of residual stress from simulation has a good consistent with that from measurment. There is an optimum laser power density for inducing a maximum surface residual compressive stress when the duration of laser pulse was fixed. The surface residual compressive stress decreases as the duration of laser pulse prolongs while the laser power density is fixed and the pulse duration is over 45 ns; The surface residual compressive stress also increases as the size of laser beam spot increases when laser power density and duration of laser pulse are fixed.