Abstract: Half-dry stroke is a kind of special effect of Chinese calligraphy during rapid moving dry brush. It is difficult to simulate because of the complex and random character. Based on the iteration function system, a novel dry brush model for calligraphy was proposed to simulate the random white effect in dry brush. This model constructed iterated function system based on user's brush speed and contrail, and rendered the attractor by chaos game algorithm as final dry brush stroke. Different from previous physics-based model, this model rendered half-dry stroke well for its random nature.