The differences of development cycle between product serial and concurrent development process are obtained through simulation in this paper, which implies the importance of rational planning in the process of product development. The models of product serial and concurrent development process are built based on witness. Three types of models for both simple and complex products have been made respectively to simulate ,and then compared results have been got under six different conditions . Among six situations, the results of four are that the time of concurrent process is much shorter than that of serial process; one is that the time is very close; another is that concurrent process is much longer. We draw a conclusion that product development process must be rationally planned in order to shorten the development cycle for some complex products.