Abstract: In this paper, direct simulation Monte Carlo method with simplified Bernoulli trails collision scheme was used to simulate the transient process of gas detection in a micro cavity(0.001 cc). The results show that the pressure in the sampling chamber decreases exponentially. The pressure change of the sampling chamber has little effect on the conductivity of the gas mixture in the capillary. The partial pressure of the ionization chamber decreases approximately linearly when it reaches the maximum value. During the simulation, the partial pressure ratio of the gas in the ionization chamber changes greatly. Moreover, there is a big error in the gas partial pressure ratio between the ionization chamber and the sampling chamber. A method to correct the gas partial pressure ratio by using the flow conductance of each component of the gas mixture is presented in the paper. The modified partial pressure ratio is in good agreement with the theoretical value.