The method of laser shock processing (LSP) is used to investigate the effect of LSP on the grain structure, micro-hardness, and residual stress of the AM50 magnesium cast alloy along its depth direction. Results indicate that the micro-hardness and residual compressive stress of the alloy surface both have significant improvement after single LSP. On the LSP layer where original coarse grains are refined obviously, the surface micro-hardness value is increased by 19%, and the residual compressive stress is up to -225 MPa. Besides, the depths of the micro-hardness-improved area, grain-refined layer, and residual compressive stress layer are obviously increased. When the impact times is increased to two, the micro-hardness, grain size, and residual compressive stress are further improved.