A two-step Lagrangian particle tracing method was used to simulate the particle movement in convective diffusion. Firstly, the flow field distribution was given by finite difference method. Secondly convective diffusion was simulated by particle tracing method. Jet current through centre slit in the static flow and turbulent flow as well as the side discharge of pollutant were simulated as case study. In this paper, finite difference method is used to solve fluid velocity field distribution, and particle tracking method is used to solve convection-diffusion equation. The result shows that the data produced by particle tracing method is reasonable and the method is stable. This method can be applied to the calculation of the convective diffusion of pollutant and spread to other problems.