Abstract: To elucidate the formation mechanism of the vortex structure in cavitation flow, the unsteady cavitation flow field of the centrifugal pump was investigated by simulation. The two-dimensional vortex structure on the mid-section was examined based on the method with velocity components. The vorticity on the blade surface and the evolution of the periodicity was compared. In addition to investigate the three-dimensional vortex structure in the impeller passage, the new Omega vortex identification method was also utilized. The results show that: as the NPSHa decreases, the cavity coverage area grows progressively, and the local vortices emerge and the flow in the pump becomes disordered. Due to the effect of cavitation flow field, the vortex structure is mostly formed and evolved on the blade suction surface. During the critical cavitation, the vortex is also detached the blade surface as the cavity rolls up. At the effect of rotor-stator interaction, the blade trailing vortex sheds and attaches to the volute wall at the front end of the tongue. According to the Omega method, the structure of the three-dimensional vortex within the cavity is stable. At the critical cavitation condition, the structure of the three-dimensional vortex downstream of the impeller passage is fragmented and disordered.