In allusion to axial flow pump and mixed-flow diffuser pump that are widely used in large low-head pump stations, the formulas for mechanical efficiency, hydraulic efficiency and volumetric efficiency of pump were given academically by analyzing the similitude of the loss in pumps. New efficiency formulas of pump and pump system were deduced. The coefficients for efficiency of pump have been obtained by using least square method to fit the experimental data of model pump performance. The losses related to shape, hydraulic friction and strike inside pumps were all considered in the efficiency formulas and the conversion methods, so they could be well applied to the design and off-design condition. Thereby the dynamical performance of prototype pumping installation could be well and truly predicted.