From the view of energy change, the decision method of frequency security emergency control is studied. Based on the relationship between the frequency change and the change of kinetic energy stored in the rotating masses of the synchronous motor, the frequency security constraint is changed into the constraint of the kinetic energy stored in the rotating masses of the synchronous. And then, based on the relationship between the variation of kinetic energy stored in the rotating masses of the synchronous motor and the energy accumulated by imbalance power of power system, the mechanism of frequency security emergency control is studied. The energy accumulated by imbalance power is controlled to achieve frequency security control objectives. Therefore, controlling imbalance power in the traditional method can be replaced by controlling energy during the dynamic frequency variation process. The flexibility of frequency security emergency control decision is dramatically increased. By simulation analysis of the real power system, the conclusion is evaluated. This work is supported by Science and Technology Project of State Grid Corporation of China “Research on Grid-Source Coordination Frequency Stability Control Techniques of Multi-DC/AC Transmission System in Large-scale Energy Base”.