Abstract: Performance evaluation is important in selection, improvement, and design of computer systems. Simulation and queue model can be used to analyze the performance of computer systems. It is expensive to create and use simulation model. Queue model needs more hypothesis and simplification. Aiming at the shortcomings of the two models, a finite state transition graph （FSTG） model based on chart and automaton theory is presented to analyze performance of computer systems in this paper. An algorithm based on FSTG for solving the response time of real time systems is also presented. By proving and complexity analysis, the algorithm is correct and effective.