The rotary pressure exchanger (RPE), one of the most important pieces of equipment in the recycling of brine pressure energy in the reverse osmosis desalination process, contributes greatly to reducing power consumption and system operation cost. In this work, three-dimensional geometric models of five kinds of RPEs with diffierent sizes and structures were built. The sliding mesh technique and species transport equations were used in the simulation process. FLUENT software was used to analyze the characteristics and factors infiuencing the concentration distribution at a rotor cross section on the seawater side after the model was enlarged and the number of rotor rings was increased. Moreover, the structure of the endcover was changed from the original two-port to the current four-port. Simulation results revealed that the model size, rotor ring number, and endcover structure affiected volumetric mixing. This research will provide theoretical guidance and innovative ideas for the design and structure modification of RPE devices.