The air leakage of forced ventilation pipe has great influence on the gas distribution in the excavation of gas tunnel. Based on Hejiapo high gas tunnel, the computational fluid dynamics (CFD) method was applied to simulate the ventilation of gas tunnel at the condition of air leakage. The area as well as position of air leakage on the gas distribution was analyzed. The results show that the jetting action leads to little gas accumulation along the ventilation pipe near the working face. The eddy effect causes large amount of gas gathering between working face and pipe outlet. The upper eddy effect and lower jetting effect of air leakage get larger with increasing of hole area, which cause the upper gas accumulation and lower gas dilution of air leakage much more obvious. The former two indexes increase linearly with the hole area. The upper gas accumulation gets obvious when the air leakage position is near the working face, when the gas accumulation decreases exponentially with the axial distance. While, the lower gas dilution of air leakage becomes larger with the increasing of axial distance. The decrease degree of lower gas increases exponentially with the axial distance. The research results provide some theoretical reference for gas monitoring and sensors layout during the excavation of gas tunnel.