In order to describe and plan the overlapping activities of product design process, a dynamic simulation model of overlapping activities is established with 3D system simulation software Simio, which is based on a mathematical model, including an information model, an information exchange strategy and a single-objective decision model. Within the simulation model, the executing process and results of overlapping activities are contrasted by setting the start time of downstream activity to different values, in order to search for the optimal time to start the downstream activity. The validity of the simulation model is illustrated by a simulation example about a flatness prober. The overlapping activities' executing process is optimized with the simulation model based on Simio, avoiding the complicated calculation and program development as well.