To alleviate the computational complexity of power system reliability evaluation, the probabilistic model of external system static equivalence was proposed in this paper. With using the traditional static Ward technique, the equivalent parameters of extern system in the state of random failures of generator and transmission line in the extern system were calculated, and then the joint probability density distribution for equivalent injection powers and series branch impedances of the extern system was achieved by utilizing probability density estimation technique. As the randomness of extern system equivalent parameters are considered, the proposed model can improve both calculation accuracy and efficiency. The proposed model is verified through the RBTS and IEEE-RTS79 test system.