The axial flow screw-type oil-gas multiphase pump is mainly applied to oil and gas transport in the deep sea. In the process of transporting the multiphase medium, the gas volume fraction (GVF) on the gas phase changes from time-to-time, resulting in the performance of the oil-gas multiphase pump being greatly influenced by the gas phase. This paper presents a detailed analysis of the gas-phase distribution law and the vortex distribution in the flow passages within the oil-gas multiphase pump by means of numerical calculations, supplemented by experimental verification. The results show that the gas phase is mainly concentrated in the diffuser at different GVFs, and the gas phase gathering in the diffuser becomes more significant with the increase in the GVF. The gas-phase volume fraction increases gradually from rim to hub, that is, the gas-phase gathering degree increases. The maximum gas-phase volume distribution area is mainly concentrated in the area near the hub of the diffuser inlet and the middle blade height area at the outlet of the diffuser. The flow in the impeller is relatively stable under the different GVFs, while there is a large vortex near the inlet of the diffuser near the hub, and there is a backflow phenomenon between the outlet of the diffuser and the tip clearance of the impeller. The volume fraction of the gas phase near the rim fluctuates more than that near the hub because the gas phase is squeezed by the liquid phase more violently. The research results can provide theoretical guidance for the optimal design of oil-gas multiphase pump blades.