A systematic analysis is made on the model building method of stochastic Discrete Event Dynamic System (DEDS) based on generalized semi Markov process, the research approaches of DEDS based on perturbation analysis(PA) are extended, and some basic problems existing in PA is pointed out and solved. A new finite automata model defined in probabilistic space is established, and a systematic proof is made of the new model being able to unify classical finite automata model and generalized semi Markov process model.