The dynamic characteristics of the grinding machines have an impact on its machining quality and cutting efficiency. In this paper, through the analysis of the structural features of the Large CNC gantry surface grinding machine, the finite element model of structure of the grinder was established on the base of the simplified machine structure, on which carried out its dynamics analysis, obtaining the modal parameters of the orders under different mode shapes. The effect on the dynamic characteristics of machine tools was analyzed under different mode shapes, then the weak link of the surface grinding machine was given, and then how to improve the structural design weakness of surface grinding machine was discussed, all of which provided theoretical basis for improvemental design for the structure of the large CNC gantry surface grinding machine.