Abstract: "Intermittent power supply" technology can achieve significant energy saving effects for Beam Pumping Motors（BPMs） in oil field. However, unreasonable power-off time not only affects the energy saving effect, but also causes the speed of beam pumping units to exceed the limit, which may compromise the operation safety and damage or break down the system under severe conditions. Therefore, the most important issue is to accurately determine the power-off time in practice. This paper proposed the accurate power-off time determination method based on speed prediction after power-off control. Through analyzing the relationship between the balance counter and the crank position, as well as, load of the polished rod and pumping, and its movement, the characteristic of load torque varying with crank angles was firstly revealed, and the load torque equations in terms of crank angles was established during power-off and power-on operations. Then, the load torque and speed curves after the power-off control can be predicted by using the measured electrical quantities, and the optimal power-off time can also be determined directly. The energy-saving controller was developed and applied to oil fields, which saves electric energy by over 15%.