The extra-long and large highway tunnel with high gas has great risk, and ventilation is the "lifeline" of tunnel construction. Based on the Tongzi Tunnel, the first tunnel on Guizhou highway under construction, taking into account gas emission, blasting smoke, the maximum number of workers, operating machinery and minimum airspeed, the air requirements, air pressure, and fan power for forced ventilation and gallery ventilation are compared and calculated. The influence of air duct diameter on ventilation resistance is discussed. The gas concentration and wind speed of the two schemes are numerically analyzed. Finally, the economy of the two schemes is compared. The results show that: The original design of forced ventilation cannot meet the construction ventilation requirements without considering the gas emergence from non-coal strata. The diameter of the duct has a significant influence on the ventilation resistance, and the 1.8 m duct diameter is chosen in combination with the actual project. Numerical simulation analysis shows that the maximum gas concentration of the forced ventilation is always higher than that of the gallery ventilation, and the gas concentration of the gallery ventilation will enter the stable area earlier than the forced ventilation and then tend to 0. The value of the speed stabilization area of the gallery ventilation is 2~2.5 times higher than that of the forced ventilation. Compared with forced ventilation, the gallery ventilation mode has lower equipment input and operating costs and better economy. The research results are typical and can provide theoretical guidance and engineering references for tunnel construction ventilation.