Presently, hydropower is the world's largest source of renewable energy. Pump Storage Power Plant develops the rapidly because of its effective electricity storage and becomes the most part of hydropower. A pump turbine is the vital component of a Pump Storage Power Plant To obtain efficient generation, safe and stable operation of a pump turbine is pretty important. However, the existence of the hump characteristics of a pump-turbine in pump mode usually leads to operating instability. Thus it is necessary to analyze regions of the hump characteristics. In this research experimental investigation and numerical simulation are employed in order to study the hump characteristics. Unsteady incompressible turbulent flow simulations for the full pump turbine model water domain are performed using the SST k-omega to turbulence model. A refinement grid is generated, which allows the corresponding y-plus values of the runner blades, stay vanes and guide vanes less than 2 in average. Calculation results of torque in different discharges as well as head and efficiency in the small discharge regions are in solid agreement with the experimental data. The results show that there are three vortex groups which distribute in the tandem cascade passages when entering the hump region. They are equally located in the circumferential direction in the tandem cascade, and one vortex group is located in two passages of the special stay vane. The strength and range of the vortex group change with different discharges. It also shows certain instability during one runner revolution. This work can provide a basic understanding for the improvement of the stable operation of a pump turbine. (C) 2014 Elsevier Ltd. All rights reserved.