Abstract: In order to study the oil-water mixing law in the oil-water replacement of product oil pipeline and to reduce the oil-water mixture produced in the process, a numerical model of the oil-water replacement of a product oil pipeline in Yunnan was established with OLGA software. Thereby, the volume of oil-water mixture under a variety of oil-water replacement conditions was simulated, and the impact of the flow rate, the shutdown time and the position of the mixture head during shutdown on the volume of oil-water mixture was analyzed. The results show that: The flow rate has the greatest impact on the volume of oil-water mixture and increasing the flow rate can effectively reduce the volume of oil-water mixture. There is a critical value for the impact of shutdown time on the volume of oil-water mixture. Definitely, in the case the shutdown time exceeds the critical value, the volume of oil-water mixture will not increase. In addition, the influencing rule of the position of the mixture head during shutdown is mainly determined by the topography where the oil-water mixture is. Generally, the research results could provide a reference for the construction scale of the oil-water mixture tank of the intermediate station and the terminal station of a product oil pipeline.