Abstract: When a pump works as a turbine, its hydraulic behavior will be changed. In order to predict the best efficiency of a pump which is used as a turbine, in this study, using multistage centrifugal pump DG85~80 for the model, through theoretical analysis on the performance of the multistage centrifugal pump in reverse, the relationship between the best efficiencies in direct and reverse modes was summarized, and then the best efficiency in reverse mode was obtained. In next step, the multistage centrifugal pump was simulated in the reverse modes by computational fluid dynamics. Using numerical results, the characteristic curves of the multistage centrifugal pump in reverse modes were obtained. Compared theoretical results with numerical results, the best efficiency was in an acceptable coincidence with the numerical datas for turbine mode, and the theoretical efficiency at the best efficiency point was 2.74% bigger than the results of numerical simulation. In order to get the internal flow field characteristics of the multistage centrifugal pump, the relative velocity and static pressure were analyzed in different positions with the same flow or under different flow, there was a large low velocity zone on the working face of the blade at low flow, with the increase in discharge, the size of low velocity zones decreased. And the pressure drop from the first stage to the forth stage was same at the same working condition, and the pressure drop in them was bigger than it in the fifth stage.