Abstract: By using the relations between lateral Lagrangian time scale and stability and height, a three-dimensional stochastic dispersion model to simulate the dispersion process in surface layer for groud level releases is build up. The results of model computation show that (1) under stably stratified conditions this model is much superior to the model of TLY equal to constant, (2) under unstable conditions, the improvement of model Ⅰ to model Ⅱ is not very great.