The method relates to a method for relieving residual stress of a high-strength rare earth magnesium alloy thin-walled casting, and belongs to the technical field of nonferrous metal processing technologies. By the adoption of the method, the residual stress in the high-strength magnesium alloy WNG421 casting can be effectively relieved, and meanwhile the residual stress can be homogenized. Moreover, the hot isostatic pressure technology can improve the mechanical properties of magnesium alloy casting materials. As for WNG421 magnesium alloy, the strength value and ductility are both improved.