Abstract: As a novel marine energy-saving device， the Propeller Boss Cap Fins （PBCF） is often properly installed behind the propeller to boost the propulsion efficiency of the propeller. As a result， the research on the energy-saving effects of PBCF is of vital importance. In this paper， a computational fluid dynamics software is adopted to simulate the open water performance of the fins. By changing the various parameters of the fins on the back of the propeller， the corresponding changes of wakes， the pressure distribution at critical sections and other performance factors are analyzed. The results show that the different fix angles of fins significantly impact the propulsion efficiency， while the change of axial positions produces little effect. Also， by observing the diagrams of wake distribution and pressure distribution， it could be concluded that the major function of fins is to produce certain torque on the same direction as the propeller and to reduce wake speed so that the hub vortex may be weakened or even eliminated.