Abstract: Abstract: Pumped storage power station plays a more and more important role in modern society. And it is the important index to judge the national energy usage efficiency and clean energy usage. At present, the development of reversible pump turbine shows several significant features, such as high head, large capacity, high speed and so on. But there exist some problems on design, manufacture and operation of the unit. The S-shaped region of complete characteristics curve on reversible pump-turbine used in the pumped storage power station brings great challenges to the stable operation of the unit, such as low success rate when starting, difficult for mode transformation and grid-connected, as a result, it will seriously influence the operation of the pumped storage power station. So, many researchers focus studies on the problem. In this paper, the study object was the 1# pump-turbine in Baoquan storage power station. According to the contrast between experimental value and calculation of "S" curves, the calculation of CFD can reflect the basic characteristics of the unit "S" characteristics actually. With the numerical calculations of 4 modes under the condition of 2 guide vane opening, the streamline distribution was analyzed in spiral case and fixed guide vane, in guide vane and runner, and in draft tube. Some literatures and actual operations of unit showed that, the "S" characteristics appeared easily under the condition of low-head turbine mode, and it would be improved with head increase. In order to explore the reasons, with the numerical calculations of runaway mode under the condition of 10 guide vane opening, the relationship between the water head variation and "S" character was analyzed comparatively. The trend and variation rule of internal flow was grasped essentially. It pointed out that the important reason for the formulation of "S" characteristics was channel congestion caused by cortex. At the same time, it showed that the effects on unit of "S" characteristics could be eliminated or reduced through the late intervention and improvement of hydraulic design. The former core was to try to avoid S-shaped region. This method was to enlarge discharge by misaligned guide vane apparatus (MGV) and has been used in Tianhuangping pumped storage power station. But it increased the difficult of unit control and workload, and the breakdown of MGV apparatus would reduce the success rate of unit's starting, and also it would cause the flow complex in the runner. So it is a remedial countermeasure. The later method was to eliminate the S-shaped region through the change of flow regime at the period of hydraulic design and model test. Taking account of two modes, now it is difficult to select pump-turbine, and it is the study emphases in the future. The research findings of this paper can provide a technical support for unit stable operation and further study of "S" characteristics.