A simple image model, the package flow model, for fuzzy simulation of nuclear reactor system dynamics is presented. By using this model, fuzzy inference rules and their membership functions are easily obtained. The system dynamics can be approximately simulated by fuzzy inference. The method and some examples are described. The advantages of this model are intuitive under-standability, flexible modification, and simplicity. Furthermore, high-speed calculation and high reliability can be realized by using fuzzy computing hardware in the near future.