This paper presents the concept of equivalent-unit and the solution method of state probabilities for short-term reliability evaluation for generation systems. The ideas and methods are applied to evaluate short-term reliability of intercommected power systems,and the models of equivalent assistance,which can be conveniently combined with subsystem models,are propesed. The models and algorithm proposed not only solve the difficult problem of "state explosion" , but also improve the calculating accuracy. A numerical example is given to show that the models and algorithm are feasible and effective.