With the goal of saving energy and reducing emissions, a secondary-balance technology was used to reform the conventional beam-pumping unit. The output torque from the reduction gearbox of conventional beam-pumping units is usually characterized by its periodic dramatic changes. On the basis of the idea of "cutting peak and filling valley" and the theory of the Fourier-series expansion for the torque curve, the secondary-balance device is designed to slow down the fluctuations in the torque curve. The secondary-balance device, with a balance principle similar to that of the crank balance, is connected to the output shaft of the reduction gearbox and can further reduce torque-fluctuation rate and peak torque. The field-test result of the secondary-balance device shows that the root-mean-square torque is decreased by 22.7%, and the energy-saving rate of the motor reaches 6.54%.