Abstract: Estimating the generation capacity to be tripped suitably is a pre-preparation for power system transient stability emergency control. With the equations of motion of a generator, this paper defined and analyzed the acceleration energy of power system during the steady state, the fault clearing time and the moment of control actions taken, and then it indicated that the equal-area criterion was a special case of acceleration energy of power system. Based on the detailed analysis of acceleration energy of power system, a method to estimate generation critical capacity tripped was presented. Different from the equal-area criterion, this method don’t need to calculate the unstable equilibrium points, integral path determined and calculation process are simple. The simulation results show that had good prospects.