This paper established the model of clouds based on cellular automata and developed the method of how to deal with boundary grid points. It also described some of the dynamic aspects of clouds by using promoted transition rules and introduced ascending air current. These techniques could reflect the true characteristics of clouds and enhance the run-time efficiency. Multiple forward scattering model was applied to cellular automata in the illumination calculation process, and the improved phase function was proposed. The simulation results showed that proposed methods could promote realism of clouds and simulation efficiency, and implement real-time rendering of dynamic clouds and large-scale clouds.