Using CFD (computational fluid dynamics) software, the flow field of the axial flow gas-oil multiphase pump's semi-volute suction chamber in different void fractions has been simulated. The flow field velocity distribution was got and the reason for gas accumulation and vortex according to the gas distribution and slice graph were analyzed. The section area of axis and the roundness corner radian were augmented and the thickness of the outlet separator tongue was reduced in order to improve the performance of the pump. By testing the prototype pump, it showed that the pump efficiency and the void fraction were obviously advanced, which indicated the measures were effective.