Abstract: Based on the pressure boundary conditions, the DSMC method for micro scale flow was studied. Two dimensionless parameters were defined as the constraints of grid size and time step for the micro scale DSMC method. The method was verified and compared through the micro scale Poiseuille flow, and the general principle of selecting grid size and time step was preliminarily obtained. On this basis, the DSMC simulation of microchannel gas flow with variable flow cross section was carried out. The flow structures of single orifice and double orifice models and the influence of inlet pressure conditions on the internal flow were mainly studied. The results show that the channel geometry has a significant effect on the micro gas flow. The channel shrinks behind the orifice, resulting in a pressure drop, which further leads to the acceleration of gas flow and the flow separation at downstream corners of the orifice. The flow structure of the double orifice model is similar to that of the single orifice model, and the gas velocity through the double orifice is lower than that through the single orifice under the same pressure difference. With the increase of inlet pressure, the flow velocity after compression becomes larger and the size of separation zone also becomes larger.