Emergency control is an important means of system-transient-stability protection. A mount of simulations of power system emergency control show that under the circumstances of multi-swing instability, action time of emergency control has nonlinear effects on the transient stability of power system. This paper quantizes the instability parameter based on the theory of dynamic security region, and compares the transient stability at different time. It puts forward an improved emergency control strategy screening method with optimal acting time. This method can quantify the influence of time when there is risk of multi-swing instability and give the optimal moment of concentrated loads shedding and generators tripping. Simulation on IEEE 10-generator 39-bus system shows that the new algorithm can find improved emergency control strategy to decrease the vibration of the system, and minimize the cost of emergency control strategy.