Abstract bstract：To study the instability characteristics of pump—turbine at turbine braking mode，a hydraulic model Wag established based on a pumped storage power station．The SST k一ω model and SIMPLEC algorithms were used to simulate the internal flow fields and external characteristics at turbine braking
condition．Compared with the experimental data，the simulation results were examined．The results show that the head changes is small as flow rate changes in turbine braking condition．Higher rotating speed is needed to get into the braking mode as the opening of guide vanes increase．The pump turbine can be prevented from running into braking mode by adjusting the opening of guide vanes．Flow is regular and there is no vortex structure at zero moment point when guide vanes at operating opening．There have lots of periodical distribution vonexes in inlet of blades and guide vanes a8 well as the region between guide vanes and stay vanes with flow rate decreased．The vortexes in runner are caused by large forward inclina-tion of blade，while vortexes in guide vanes are due to the angle of attack between flow direction andguide vanes．These conclusions can provide a theory basis for optimal design for pump-turbine．
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