Abstract: For the vertical four-axis inner cavity machining special machine,the mechanical dynamics modeling is carried out based on the dynamic sub-structure method. The dynamic stiffness characteristics of the ball screw pair and linear guide pair of the special motorized joint are solved. The natural mode of the whole machine is analyzed by the established dynamic mode, and the natural frequency is solved by the analytical method. The finite element mode of the machine is established by the finite element software, and the analysis results of the natural frequency, mode shape and harmonic response of the whole machine with the rigid contact of the joint and the dynamic stiffness are set. The results show that the established dynamic model with dynamic stiffness can effectively mirror the dynamic characteristics of the machine, and can be applied to the dynamic analysis of the machine and the research of the digital twin virtual prototype.