Along with the development and appliance of the artificial intelligence, professional have adopted many kinds of methods such as expert system, NN and so on to explore an innovation type CAPP. The paper gives a brief introduction to the principle and design methods of process planning based on the theory of expert system developing a computer aided process planning system of sheet metal and discusses in detail the system uses the network graph to describe the parts and technology decision-making route, and the methods of separated reasoning and various expressions of knowledge are used to solve the complexity of development in CAPP research. Finally satisfied the requirement of the practicability?opening and intelligence about the system.