Under extreme working conditions, CNC machine tools are prone to oil and water leakage in the joint gaps of the sheet metal parts of their external protective devices. This phenomenon not only seriously affects the production efficiency of CNC machining, but also causes significant environmental pollution. This article uses simulation analysis method and orthogonal experimental method to study the influence of main structural parameters on the dynamic stability of the connection process of sheet metal parts of the external protective device of CNC machine tools, and further proposes an optimized connection process scheme for fixed sheet metal units and relative movable sheet metal units. The results show that the optimization scheme can solve the problem of oil and water leakage at the joint of the sheet metal unit of the external protection device of CNC machine tools, and provide reference and guidance for the structural improvement of the external protection device of mechanical equipment.