Based on particle swarm optimization（PSO） algorithm and considering N -1 security constraint， a method for available transfer capability（ATC） of power system is presented in this paper. The basic idea of this new method is to verify a possible ATC value satisfying power system static security constraint. The variables of each particle are classified into two elements: exchanged power value and the outage branch number. These two elements are evolved in different ways. In the procedure， a modifying measure for the infeasible particles is taken to accelerate the speed of finding the ATC value and the feasible particle is used to identify the severe contingency. After the calculation， the key branches and severe branch contingency set are attained as well as the ATC. Comparing with other methods， the numerical simulation results in IEEE 118-bus system demonstrate the higher efficiency and validity of the method.