Advancing direction of massive laser shock peening (LSP) impacts often generates different residual stress distribution at the surface and along depth direction of metallic sheet. To investigate the effect of advancing direction of massive LSP impacts on the induced residual stress fields of AM50 Mg alloy tensile samples, a 3D analysis model is developed by using ABAQUS software. At the same time, the difference between the double- sided simultaneous LSP and double-sided non-simultaneous LSP impacts is also presented. Results show that advancing direction plays an important role in the generated residual stresses, and simultaneous LSP impacts can induce a better simulation results than non-simultaneous LSP.