Abstract: To study the effect of tunnel section mutation on the time and temperature of smoke spreading to the air inlet and outlet, Pyrosim fire simulation software was used to establish different slope of the tunnel, length and location of sectional mutation of different tunnel sections. In addition, visibility, temperature and smoke spread measurement points are set at different spatial locations. The simulation results show that, the simulation results show that with the increase of roadway inclination, the inhibition effect of roadway mutation on flue gas spread decreases; the higher the intensity of the fire source, the higher the inhibition rate of the sudden change of roadway on the time of smoke spreading to the air inlet. However, the inhibition rate of the smoke to the air outlet time decreased. When the section length of the tunnel is 10 m and the distance from the fire source is 50 m, the effect of the section mutation on the smoke spread is the greatest.