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Cavitation is a complex multi-phase flow phenomenon. In the development of cavitation, the transient phase transition between liquid and steam results in multi-scale vortex motion. The transient cavitation dynamics is closely related to the evolution of cavitation vortex structure. Two vortex identification methods, Q criterion and Omega discriminant method, were used to explore the cavitation and vortex characteristics of centrifugal pump impeller region and their effects on pressure pulsation under different cavitation degrees at rated flow Qd=0.321 m3/s. Based on Schnerr-Sauer cavitation model, the full-channel flow field of vertical single-stage single-suction volute centrifugal pump at four different cavitation degrees, including the initial stage of cavitation, the development stage of cavitation, the transition stage of cavitation and the deterioration stage of cavitation was numerically simulated. The results showed that the flow in the impeller region was complex under cavitation conditions, and the cavitation morphology and vortex formation affected each other, and both influenced the pressure pulsation in the impeller region. The Omega method can emotionally capture the reflux vortex at the inlet of the impeller, the passage vortex in the impeller channel and the wake vortex at the trailing edge of the impeller. In the early stage of cavitation, due to the influence of large-area channel vortices and volute tongue in the passage, the size of the bubbles above each blade was different. When cavitation was severe, high-speed vorticity of gas-liquid mixture appeared under the influence of cavitation loss, resulting in the increase of low-frequency pressure pulsation signal, and the energy released by the cavitation moving to the high-pressure area led to a significant increase in outlet pressure pulsation.