Abstract: Aiming at the irregular shape and unpredictable characteristics of clouds, a real-time cloud simulation algorithm is discussed. The discrete distribution of clouds is randomly generated, and their dynamics is described by a group of Boolean rules. The continuous density distribution is computed through the Sigmoid function. We propose an improved multiple forward scattering illumination model to compute the color and intensity of clouds, and render it using ray casting algorithm on the GPU. The experimental results show that the algorithm is better in real-time and more realistic, comparing to texture mapping, particle system and fractal geometry methods.