Taking a large-scale CNC gantry milling machine bed structure as the research object,using ANSYS Design Explorer optimal design module,the thickness of the body ribs and the sand hole diameter were taken as main optimal variables to make lightweight design on the premise of same bed original strength and stiffness.The analysis results show that:after lightweight design,the maximum values of the bed deformation,stress,strain are declined and their distributions are improved,weight reduction is 11.3%.