To investigate the impact of cavitation on the unsteady flow and energy loss characteristics in a water-jet pump, the unsteady calculations of its internal cavitating flow field were carried out based on the unsteady Reynolds-averaged Navier-Stokes (URANS) equations, adopting the FBM-CC turbulence model and ZGB cavitation model. The unsteady flow and energy loss characteristics under typical cavitation conditions were analyzed by combining the boundary vorticity flux diagnosis and entropy generation theory. The results show: The adopted numerical simulation method has good accuracy for the calculation of cavitating flow in the water-jet pump; as the net positive suction head decreases, cavitation develops from clearance cavitation to wedge-shaped cavitation; under the deep cavitation condition (the net positive suction head is 11.32 m), cavitation covers almost the entire blade suction surface, inducing the fracture of the tip leakage vortex and the formation of cavitation-induced vortices that block the flow passage; a strong adverse pressure gradient exists at the cavitation end, and streamline distortion and flow separation appear at the tip trailing edge; in the initial stage of cavitation, the energy loss is dominated by wall friction; under the deep cavitation condition, turbulent dissipation becomes the main contributor, and the entropy generation of turbulent dissipation increases by 5% compared to the initial cavitation; at this time, the flow separation of the wake and the vortex caused by cavitation become the main sources of energy loss.