This paper presents an analytical method to evaluate the short-term reliability for generation system. Based on the discretization of continuous Markov Process,the transient performance of short-ferm reliability is considered, and the Loss of Load Expection(LOLE) Energy Index of Reliabihty (EIR) are calculated step by step. This method is applied to evaluate short-term rehability of a generation system,and the results of the evaluation show that the method is feasible.