Sheet thickness is an important factor affecting shock effect when metal sheet is treated by two-sided simultaneous laser shock processing (LSP). AM50 Mg alloy sheets with different thicknesses are treated by two-sided simultaneous LSP, residual stress fields of all AM50 Mg alloy sheets are simulated by ABAQUS software. Influence mechanism of two-sided simultaneous LSP on metal sheet as a function of sheet thickness is systematically investigated, then residual stress distributions in the axial and radial direction are analyzed. Results show that under the same laser spot diameter, pulse width and peak pressure, with sheet thickness increasing, compressive residual stress in the model becomes more uniform and regular, and the influence depth turns deeper. When the sheet thickness reaches a threshold, both of them reach saturation. The appropriate thickness of AM50 Mg alloy sheet treated by two-sided simultaneous LSP should be 4 mm and more, when peak pressure is 1600 MPa, pulse width is 57 ns and laser spot diameter is 3 mm.