A threedimentional model of highway tunnel is established and numerically simulated by means of hydrodynamics software Fluent. The gas distribution in main tunnel and connection gallery and at working face are studied; and the gas distribution characteristics in tunnel, especially in recirculating zone, eddy zone and gas concentration zone, are analyzed. The simulation results show that: 1) The gas gushes from a main tunnel has no harm to another main tunnel. 2) The gas concentration in eddy zone is larger than that in surrounding zones. 3) The gas concentration at the side with ventilation pipe is smaller than that at the side without ventilation pipe.