A Lagrangian equation of v'_giv'_pi on the particles trajectory is obtained with stochastic process theory, and then Lagrangian equation of particle Reynolds stresses is exactly set up. Wang Stock experiment on heavy particle dispersion considered scale ratio and flow decay is simulated by both this method and Zhang's model^[1]. Simulation results show the new method is better than Zhang's model. A backward-facing step two-phase flow is calculated by the new model and the results show good agreement with the experiment results.