Abstract: A beam pumping system is of the following characteristics: 1) the rotating speed of the motor changes instantaneously; 2) the load torque of a belt-driving unit fluctuates greatly; 3) the longitudinal elastic deformation of the belt changes instantaneously. With the belt simplified as two springs of no mass, and with the motor speed fluctuation, the sliding friction between the belt and the pulleys as well as the influences of the belt transmission’s instantaneous load torque on the pulley sliding angle and equivalent spring stiffness taken into consideration, a dynamic simulation model of a belt driving unit of a beam pumping system is built. According to the simulation results of the driving-driven pulley’s momentary speed and the instantaneous output torque of the motor, the simulation model of belt instantaneous transmission efficiency of beam pumping units is established. Simulation results show that: 1) the motor rotating speed fluctuates periodically, and the higher frequency fluctuation of the motor rotating speed is made by the belt’s alternating longitudinal elastic deformation; 2) there is a rotating variation loss between the driving-pulley and the driven-pulley, and the belt transmission efficiency is reduced by the driven-pulley speed loss; 3) after the system achieves a stable running state, the transmission efficiency of the belt unit changes periodically, and its varying amplitude depends on fluctuating characteristics of the load torque. Average transmission efficiency is decreasing with the load torque increasing, and it is also decreasing with the torque balance degree decreasing; 4) belt’s average transmission efficiency of a beam pumping unit is usually less than 90%. When the load torque is bigger and the torque balance degree is smaller, it is possible that the average transmission efficiency of the belt driving unit is less than 85%.