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Aiming to investigate the formation mechanism of highspeed water ring in the vaneless region and the effect on the stable operation of the unit under the runaway condition, the pumpturbine of one certain pumped storage power station was employed. Based on realizable k-ε turbulent model, the unsteady flow of the whole passage of pumpturbine was calculated. Some parameters such as the speed of the highspeed water ring were studied in bladeless zone. And comparing the numerical simulation results with the test, the results showed that there was an intrinsic connection between the Sshaped curve of the pump turbine and the stability of runaway operating. The value of fluid velocity in the bladeless zone was larger under the runaway condition. Highspeed water ring in the bladeless zone under runaway conditions led to instability of running under the runaway condition, the highspeed water ring became more obvious with the decrease of the opening. Under small flow conditions, due to the large angle of attack between the incoming flow direction and the blade bone line, the vortex structure of the runner blade ends was regularly developed, and the angle of attack was increased with the flow variation. Vortex evolution was random and vortex evolution affected the change of blade angle of attack. The angle of attack varied greatly over time and there was great randomness. Highspeed water rings were responsible for this random fluctuation.