Abstract: The coordination of preventive and emergency controls can greatly reduce the overall coordination cost, release the instability risk and meanwhile poses computational challenges for real application in a real power system. With the introduction of a system transient security index, the coordination problem in this paper was decomposed into a preventive control sub-problem considering system transient security constraint and several emergency control sub-problems considering independent transient stability constraints. A bi-level optimization model for the coordination problem was formulated accordingly. On the upper level the total coordination cost was minimized by a coordination parameter adjustment; and on the lower level the preventive control and the emergency controls were performed successively to enhance the transient security and the transient stability performance, respectively. Finally, a hybrid method that combines the golden section search and the successive linear programming was proposed to solve the proposed model. The effectiveness of the proposed method was demonstrated on the New England 10-generateor 39-bus system.