Large gantry machining center can be applied to large diameter and thickness of the flanges, tube sheets and other large sheet metal processing, the industrialization of this type of device for improving the development of modern processing and manufacturing of great strategic significance. The design and optimization of large-scales heavy gantry CNC Machining Center was mainly investigated in this paper. The finite element model of the beam structure was structured by using finite element analysis software-ANSYS. On the basis of analysis results, the optimal static and dynamic performance of square cross-section of the beam structure has been obtained. The maximal displacement is 0.531 mm. The maximum displacement of X=0.0329mm, and Y=0.531 mm occurred in the contact point of middle beam and spindle box. Z is 0.0948mm. The maximal displacement of Y-component is occurred in the contact points of guide and spindle box. This may have a certain impact on the machine processing accuracy. In the middle of the beam can consider to strengthen its internal structure, such as adding reinforcement measures to further improve its rigidity, and improve the machining precision of the whole machine.