Abstract: In order to study the influence of different fire source positions on fire smoke spread and fire parameters under normal ventilation, the distribution of smoke flow, temperature, and CO concentration in the tunnel under three conditions of single fire source at the roof, single fire source at the floor, and double fire sources at the roof and floor was simulated and analyzed. The results concluded that in terms of smoke thickness, the downstream settling of smoke was significant for fire at the floor and fire at both roof and floor, but no upward plume was observed for fire at the roof, and the smoke flow is more concentrated in the top. In terms of the backflow distance, the fire source in the floor is the shortest, followed by the fire source in the roof, and the fire source in the roof and the floor is the largest; the main influencing factor is the maximum smoke flow temperature difference in the roof. In terms of temperature distribution, the roof smoke flow temperature decreases with the increase of the distance from the fire source, and the fire source is located at the roof and floor at the same time. For the upper roof smoke flow temperature, the roof and floor fire is the largest and the floor fire is the smallest, while the downstream is completely opposite. In addition, the smoke flow temperature of the roof fire at 2 m height is lower than that of the other two conditions. For the CO volume fraction at the height of 2 m, the peak concentration of roof fire and floor fire appears at the front of the fire smoke, while the floor fire appears near the fire source, and the CO volume fraction upstream of the fire source is significantly higher than the latter under the former two working conditions.