This paper proposes a novel optimization method of transient stability emergency control based on a new concept of the so called extended practical dynamic security region (EPDSR) defined in this paper and four experiential laws about the EPDSRs found from a number of studies in real power systems. In this method, the effect of a control action is represented by the displacement of EPDSR's critical hyper-plane boundary on the direction of its outer normal vector. If an unstable contingency occurs, appropriate emergency control actions are triggered so as to the enlarged EPDSR can cover the current operating point. Based on these ideas, a mathematics model of emergency control strategy is developed for minimizing its total cost and guarantying power system transient stability. The simulation results on the 10-generator, 39-bus New-England Test System as well as other real power systems have shown its validity.