When milling large diameter internal thread parts, it is often due to the selection of improper milling cutter diameter and cutting parameters, resulting in after cutting, the surface shape error of the workpiece is large and the machining accuracy is low and other situations. Tool diameter and cutting parameters affect tool path to different degrees. Thus the theoretical roughness of the machined surface is further affected. Therefore, it is of great importance to study and theoretically analyze the influence of tool diameter and cutting parameters on tool path. On the one hand, the tool path matrix equation is set up by using the homogeneous coordinate transformation method. According to different reference objects, the matrix equations of two kinds of point trajectories on cutting edges are established. On the other hand, single factor test method is used to select different tool diameters and cutting parameters for machining trajectory simulation. The results show that with the increase of milling cutter diameter, milling cutter speed and the number of milling cutter teeth, the theoretical roughness decreases, making the machined surface close to the ideal smooth surface.