Based on the general OPF model and multi-machine power system classical mathematic model, by using the implicit trapezoidal rule, all swing equations will be converted into numerical equivalent algebraic equations set which involved in equality constraints set, and all rotors' angles related to COI are used to be inequality constraints. According to the above, a new OPF module is proposed to calculate the available transfer capability with transient stability constraints. Using primal-dual interior point method to analyze the model, a nonlinear complementary function is proposed to overcome the disadvantages which in each iterations, the primal-dual interior method’s complementarity relaxations must keep positive direction. Calculation result of 14-bus power system proves the methods effectiveness.