Abstract: Underground gas pipe gallery is a semi-closed space. Once leakage occurs in the gallery, it is likely to induce serious explosion accidents. Safety assessment of pipe gallery after leakage can provide theoretical guidance for emergency treatment of such accidents. Therefore, according to the structure of a domestic gas pipe gallery, modeling and solution with CFD software were conducted for leakage at the most unfavorable point of the gas pipeline. The variation of mass concentration of methane in the pipe gallery under different leakage diameters and ventilation modes was studied, the explosion hazard area was divided within the pipe gallery, and the safety of pipe gallery was assessed. The results show that for the leakage diameters of 10 mm, 50 mm and 100 mm, after the mass concentration field of methane in the pipe gallery becomes stable, the mass concentration of methane at the top of the pipe gallery in the accident ventilation mode is reduced by 50%, 45% and 36% respectively, compared with that in the normal ventilation mode. In the case that the ventilation volume is constant, the enlargement of the leakage diameter will increase the leakage volume of natural gas, but it will not necessarily lead to the expansion of hazard areas, and a smaller leakage diameter may bring more potential safety hazards. When the leakage is large, more serious consequence may be resulted in if the proportion between the air volume and the leaked gas volume cannot be controlled properly. The key to deal with the gas pipeline leakage in pipe gallery is to control the proportional relationship between the ventilation volume and the methane leakage volume.