In order to investigate the effect of waterway arrangement on cooling efficiency, an empirical formula between the number of waterways and cooling time was established. Based on the design of the water-cooling system under traditional processing conditions, the paper takes polypropylene (PP) sheet injection molded parts with a size of 200 mm×100 mm×2.24 mm as the research object. NX UG10.0 was used for 3D modeling of PP sheet injection parts, and then Moldex 3D analysis software was used for finite element analysis of different waterway number and spacing combinations. After comparing the cooling time calculated based on the empirical formula of cooling system design with that simulated by Moldex 3D analysis software, the empirical formula between the number of waterways and the cooling time is derived. The formula takes the number of waterways and the distance between waterways as independent variables, and the cooling time as dependent variable. The results show that under the premise of considering only the number of waterways and the distance between waterways,when the number of waterways is 30 and the distance is 22 mm, the expected requirement is best met. At the same time, it is found that the change of effective heat transfer area has a significant effect on the cooling efficiency, while the change of waterway distance has no significant effect on the cooling efficiency.