The electrical machinery power factor and the rotational speed are unable the real-time measurement which is restricted by the condition on site, so the motor output power also can't calculate accurately, which leads to difficulty to calculate the pump efficiency. This paper considers pump's differential pressure, pump flow, the liquid viscosity and other parameters, which influence on the calculation results. Then we present a new calculation method of efficiency for twin-screw pump. We program the simulation computer to calculate and analysis the pump efficiency with the calculation method. The results show that this method is simple and the error is less than 3%. This calculation method can meet the needs of engineering calculation.