Abstract: Cavitation is one of the key factors that reduce hydraulic efficiency and service life of pump impeller. This study has conducted full-passage CFD simulations of the unsteady gas-liquid two-phase flows in a mixed-flow pumping system to optimize the impeller and improve the pump performance against cavitation, focusing on unsteady cavitation characteristics at operating flow rate in the range of 0.5Q ~1.0Q, spatial distribution and time variation of cavitation, and effects of different net positive suction head (NPSH) on cavitation volume fraction. The results show that cavitation inception occurs at the inlet edges of blade suction sides and cavitation of higher volume fraction gathers near the inlet edges of blades. In the conditions of NPSH lower than a certain threshold, impeller channels will be dominated by cavitation. As the operating flow rate is reduced, the NPSH threshold is decreasing, but serious cavitation is developing faster and cavity content is increasing exponentially.