Abstract: To research into the cavitation characteristics of pump turbine and induced flow instability, a method of cavitation numerical simulation in combination with model tests was used. The cavitation characteristics of pump turbine under pump conditions with different flow rates were obtained, the structural characteristics of turbulent field in pump turbine under pump condition with different cavita-tion coefficients were explored, the correlation between cavitation and vortex in pump turbine was clarified, and the cavitation flow characteristics of pump turbine and its mechanism of interference instability with mainstream were revealed. The results show that under pump conditions with different flow rates and cavitation coefficients, the location and intensity of cavitation occurs with obvious differences. Under pump condition with small flow rate, the flow separation caused by the intense cavitation on the suction surface at the low-pressure side of the blade interferes with the main flow, and large areas of decurrent and eddy current are produced, which reduces the efficiency of the pump turbine. Under optimal operating condition and large flow rate condition, once the flow separation caused by cavitation at different positions of adjacent blades interferes with each other, a large range of backflow and eddy currents will be formed in the runner blade passage, resulting in a significant reduction in the efficiency and head of the pump turbine.