Abstract: As super-span cross-rail of NC heavy-duty planer type milling machine tool has a super span and heavy mass,it will bend down after being installed with a heavy mass and the force caused by bottom slid and quill. The largest deformation of Z-guided face attains 1 mm,which seriously influences the machining accuracy. Machining an arch camber in advance is used to compensate for the influence caused by deformation of the cross-rail. Finite element method is used to analyze the deformation of the super-span cross-rail when working. Multinomial is used to fit the deformation curve based on deformation of the Z-guided face of the cross-rail when the bottom slid and quill is at any position. This paper takes into consideration the impact on camber curve as the bottom slid's span brings about two deformation curves. A method based on the two deformation curves is put forward to design and optimize the camber curve. By comparing various camber curves' effects on the deformation of the milling head,results show that the optimization method can effectively reduce the influence of cross-rail's deformation on the machine tool's accuracy.