The load of beam pumping unit is changeable, which is often in a state of light load. Reducing a certain voltage can improve the power factor and efficiency of the beam pumping unit when in light load .We can change the voltage by changing the thyristor trigger angle. It is complex and unacceptable to analyze the change of the cycles of the load overall. So we can divide the load of the whole cycle into several equal parts, each can be thought of as a constant load. The most optimal voltage for the current load can be calculated by genetic algorithm. When each load is in the most optimal voltage, we can get the whole optimal voltage changeable rule. Then it produces the result of energy saving.