The complete characteristic curve of the pump turbine show "s" characteristic in the anti-pump and pump working-condition. The multi-numerical values of unit discharge and unit moment the curve caused by crossover and overlap phenomena of the curve may bring inconvenience for transition process analysis. In this paper, complete characteristic curves of the pump turbine were dealt with using improved Suter curve transformation method. A surface fitting were carried out for transformed WH curve. Therefore, a three-dimensioned surface was achieved. The transition process of load shedding was analyzed using the curve above-mentioned. The results show that the method used in the paper can eliminates the uneven distribution, crossover, overlapping and multi-value characteristic, thus providing convenience for the transition process analysis of pump-turbine.