Abstract: With the great development of social economy, more and more mechanical parts with complex surface are adopted. The traditional theory and methods system of cutting forming for mechanical parts based on concept of generating line can not satisfy the needs of machine tool motion analysis, motion control and processing quality control, so it is necessary to build a new theory and methods system of cutting forming for mechanical parts with geometric methods. Firstly, the traditional theory and methods system of surface cutting forming for mechanical parts based on concept of generating line is analyzed. The shortcomings of the traditional theory and methods system are discussed, and it has pointed out that the traditional theory and methods system can not fully and rigorously and illustrates and precisely the essence of surface cutting forming for mechanical parts. Then, a methods system of surface cutting forming for mechanical parts based on geometry and tool classification is proposed. The new methods system includes: (1) Using a non-rotating tool, the work-piece surface is formed by motion scanning of the cutting edge curve; (2) Using a rotating tool, the surface of the work-piece is the envelope of the surface family formed by the movement of the tool surface. The new theory and methods system can fully and rigorously and precisely illustrates the geometry essence of surface cutting forming for mechanical parts. The requirement of tool path control of NC cutting complex surface and solution of work-piece surface is met, it is applied more and more widely. Finally, for NC milling Surface, the solution method of tool path and work-piece surface is given spherical milling tool.