To deal with the lack of detailed depiction of neutral gas in PIC simulation, the paper presents a two-dimensional model of multi-element and multi-phase neutral gas to depict explicitly the fields of neutral gas in plasma region. Numerical methods of solving the proposed equations as well as various boundary conditions are put forward. The above model and method are applied to the simulation of adiabatic gas diffusion. The results obtained well accord with the theory. The model can be used in improving PIC simulation programs.