Abstract: Considering the time-varying dynamic characteristics of the workpiece of slender shaft parts and its coupling relationship with the dynamic characteristics of the tool, an orthogonal turning and milling dynamic model was established, the time-domain discrete method was used to analyze the cutting stability of orthogonal turning and milling, and the three-dimensional stability leaf lobe diagram considering the spindle speed, axial depth of cut and cutting position was established. The accuracy of the three-dimensional stability leaf lobe diagram was verified by using the time-domain simulation method. To optimize the milling tool to achieve the purpose of improving the orthogonal turning and milling efficiency of slender shaft parts, the stiffness of the milling tool was taken as the optimization objective, the global ultimate depth of the cut of the three-dimensional stability diagram was maximized, and the maximum and minimum optimization algorithm was used to obtain the best tool stiffness, which provided a theoretical basis for the optimization of orthogonal turning and milling tools for slender shaft parts.