Based on the assumption of tiny bubbly flow, the gas-liquid two-phase flow in a multiphase rotodynamic pump was numerically simulated with two-fluid model in both steady and unsteady modes. In the simulation, the inlet gas volume fraction was 15%. The k—ωbased SST model was used for turbulence. The drag force and the added mass force were accounted for in the interfacial momentum transfer terms. Through the analyses of the distribution of gas volume fraction and velocity vectors, the unsteady flow characteristic in the multiphase pump was explored. The results demonstrated that a phenomenon named “discontinuous air mass movement” would occur in the two-phase transport process. The formation of this phenomenon was related to the bubble size and the interfacial forces acting on the gas, and only in unsteady simulation could this phenomenon be observed. Also, it was found that the locations of gas vortexes in meridional plane were almost fully corresponding to the regions of high gas volume fraction, which illustrated that gas vortex was one of the main factors that led to gas accumulation. In addition, through comparison of the external characteristics between the simulation and the experiment, the reliability of the numerical method was validated.