The process of transmission network planning considering the electric power system security is designed in this paper. It has renovated the traditional planning method in which only N secure rule and N -1 secure rule are considered， and the load shedding costs of the planning network in N -2 condition are further analyzed. The fault selection step is first carried out so that the computing of the N -1 security network planning can be speeded up and the large real network planning can be realized meeting the demand of engineering applications. Tests on the 18-bus system and a real power network of 147-bus system show that the method proposed is effective.