Abstract: The transient start-up characteristics and induced cavitation instability of mixed-flow pump are the urgent problems that should be solved in the development of pump-jet system and submarine weapon towards large-scale and stabilization. The transient start-up performance and cavitation flow field characteristics of a mixed-flow pump were studied by means of pump hydraulic characteristic test and high-speed photography visualization technology. The results show that the start-up duration and flow rate do not affect the pump performance under steady operating condition if there is no cavitation appears during the start-up process. The pump flow rate, speed and head reach the steady-state peak at the same time, showing a good synchronicity under different start-up conditions. However, the variation of pump head during the early stage of start-up lags behind the shaft speed and flow rate, and the retardation time of head is 50% of the start-up period at design flow rate. Under the condition of cavitation start-up, the cavitation inception first appears at the tip clearance of mixed-flow pump. With the increase of start-up time, the cavitation could develops into a long strip shape. With the decrease of inlet pressure or increase of inlet flow rate, the cavitation could gradually presents a triangle-shaped, and finally blocks the channel between impeller blades. The evolution of cavitation morphology in the flow passage of mixed flow pump under different start-up conditions is captured through high-speed photography experiment. It is found that shortening start-up duration, reducing the start-up flow rate and increasing start-up pressure at the pump inlet are the effective ways to suppress the occurrence of start-up cavitation.