In this paper, a method of combining particle system with density fields is presented to simulate smog movement and diffusion . By attaching a density function to the particle attributes, smog diffusion is simulated through the variance of particle effect radius and density functions, effect of turbulence is imitated through particle movement and fragmentation. Compared with the traditional particle system, this method can describe smog diffusion and produce continuous density fields with less particles, the computation is thus greatly speeded up. A quick algorithm for smog rendering is also presented for real-time simulation.