To show the whole process of the power system which is a hybrid system when it’s under-frequency load shedding(UFLS) device acts more clearly、intuitively and easily, solve the problem of the simulation process when linking and harmonizing the frequency and the judging part of the UFLS, improve the efficiency of the power system UFLS simulation and analysis,then use the Matlab/PSB and Stateflow to simulate the three-machine power system and the judging part of UFLS respectively. Propose a scheme that applying the Stateflow into the complex power system UFLS simulation, to achieve the application of the stateflow into the power system UFLS, and then use an example to simulate and analysis, the result proves that the application of the stateflow into complex power system UFLS is rational and effective.