Abstract: The direct simulation Monte Carlo method was used to study the collision process between the molecules when the pure and binary gas mixtures are in equilibrium.The mean free path and the number of collisions per unit time and unit volume are calculated by using variable hard sphere model and variable soft sphere model between 300 K and 20 000 K.In order to check the accuracy of the simulation of the two models,the results are compared with the numerical calculation results under the corresponding models.The results show that the relative deviation of the simulation results of the pure gas and the calculation results does not exceed 0.7%,and the relative deviation of the mixed gas does not exceed 1.5%.The simulation results under the above two models are compared with the calculation results under the hard sphere model.It is found that under the hard sphere model,the mean free path of gas molecule has a larger error than the other two models at a wide range of temperatures,and the number of collisions gas molecule in unit time and volume is proportional to the square root of the simulated temperature.When the simulated temperature is higher than 300 K,the results are bigger than those of the other two models.