Pumped storage units are the main parts in China\'s power construction,as a hot issue concerned by the industry.The pump turbine involves the two-way flows and a multiple condition operation,and its operation flow pattern is very complex.The particle image velocimetry (PIV) is a very effective test technique to determine the internal flow field of pump turbines.This paper discusses the key problems of the pump turbine,based on the PIV experiments under typical conditions of the pump turbine,especially for problems such as the S-shape problem,the hump problem,the pressure fluctuation problem and the cavitation problem.In the internal flow fields under typical conditions are determined.The vortices induced and their development are observed in the PIV test.The flow phenomenon is shown at each operating point.The typical problems of the pump turbine are closely related to the vortex distribution in the internal flow field.From the PIV test results under several working conditions and from the comparisons between the optimal condition and the part load condition,it is seen that the vortex distributions are very different.Vortices at the vane-less area between the guide vane and the runner are closely related to the strong pressure pulsation,the first hump and the S-shape curve.From the PIV results of the cavitation working points,it is seen that the flow angle is changed in the vane-less region and the runner leading edge because of the cavitation bubbles and that the flow angle deviates from the optimal setting angle.From the computational fluid dynamics (CFD)result of the second hump working points,it is concluded that the vortex shedding on the runner leading edge is the main cause of the second hump.