The complete characteristic curves of pump turbines appear "S" type at both pump and anti-pump working conditions. The multi-value characteristic of unit discharge and unit moment of the curves due to crossover and overlap phenomena can cause inconvenience for transition process analysis. Applying the method of improved Suter-transformation, the complete characteristic curves of the pump turbine can be transformed into two groups of periodic variation curves in the four quadrants, namely the WH(x,y)and WM(x,y)curves. The complete characteristic curves of the pump turbine were carried out for transformation based on the same curve characteristic of existing data, then the transformed fitting WH curve and WM curve were obtained by adopting the mathematical model of least square surface. Therefore, a three-dimensional surface was achieved containing the two curves. The WH curve and WM curve after transformation using improved Suter-transformation are smoothing and approximately parallel to each other, and therefore the multi-numerical values and nonuniform distribution defects of Suter-transformation curves can be avoided. So, it is possible to eliminate the crossover and overlap characteristics under the pump and the reverse-pump conditions, and improve the nonuniform distribution phenomenon at the two ends of the pump-turbine complete characteristic curve, thus it is conveni-ent for the transition process analysis of pump-turbines.
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