Axial thrust is one of the critical factors that affect the pump’s continuously operating reliability. Among all the available methods for axial thrust controlling, Pump Out Vanes (POVs) are an easy and effective way. Different from a single-stage pump with a scroll, an in-line multistage pump will have a leakage flow channel from the return channel. With this leakage channel, the working environment of the POVs will be significantly different from a single-stage pump. In this paper, the first stage of a multistage pump with both POVs and casing ribs (vortex breakers) is studied by CFD simulation to evaluate their effect on the axial thrust, pump stage performance, and the internal leakage flow. Because of the similar POV working environment in the multistage pump, the conclusion from one stage can be generalized for the rest stages. In this study, 5 models with different POV outer radius and height are simulated in Ansys Fluent with k-ε turbulence model and transient rotor-stator sliding mesh method. The results show that POVs with suitable geometry can provide good axial thrust control over a wide pump operating range while the stage efficiency can be strongly affected due to the increased turbulence and interstage leakage flow, which is contradicting some previous researcher’s conclusion based on the study of a single-stage centrifugal pump.