Abstract: To study pumping well efficiency calculation is one of the effective ways to improve system efficiency. In view of the tedious testing and calculation of pumping well efficiency in Chinese oil fields, the author has obtained satisfactory results with computer and downhole diagnosis of pumping well. On the basis of phase diagram of the electric motor, the mathematical model of the variation of electric power and current with pumping unit torque and transmission efficiency of beltreducing gear is derived. The results of downhole diagnosis; surface dynamometer, pump dynamometer, pumping unit crankshaft torque, torque curve and number of strokes are processed with a BASIC program to give 27 parameters of the pumping well, electric power and current curves as well. Computer simulation is used to calculate the transmission efficiency of belt-reducing gear. The calculated values are in agreement with measured ones. This method is based on S=S (M,η₂). Together with the mothematical model, this method has combined computer diagnosis and pumping well efficiency calculation, which not only enhances the diagnosis technique but also provides an effective way of efficiency analysis.