In order to solve the problem of machining deformation of weak rigidity and slender beam with casting titanium alloy ZTC4,the machining process was analyzed by using finite element simulation method, and the experimental verification was completed according to the deformation data predicted by simulation and the method of mirror compensation processing. Through the establishment of a threedimensional finite element simulation model of slender beam CNC side milling, the deformation range of the part body at different positions was predicted, and based on this, the numerical control milling machine was used for experimental verification. The results show that the noncompensation processing method will lead to the deterioration of the surface quality of the parts, and the flatness and straightness of the parts surface after compensation processing are greatly reduced, and the reduction ranges are 17% and 33%, respectively. Therefore, it is determined that the compensation processing method can be used to effectively reduce the shape error of the weak stiffness structural part and improve its machining accuracy.