The geological setting of Sichuan-Tibet Railway is complicated, while magmatic and metamorphic rocks are widely distributed. Reference to the other engineering experience of underground works under similar geological conditions at home and abroad, the analysis shows that it exists the hazardous possibility that the inorganic gas are toxic and harmful in the tunnels of Sichuan-Tibet Railway. Combining the conditions that inorganic gas produced and the geological conditions along the Sichuan-Tibet Railway, it analyzes and predicates that CH4, CO2, H2S, SO2, CO, NO2, NH3 and H2 are the main types of poisonous gas along the Sichuan-Tibet Railway. The poisonous gas is mainly distributed in the three suture zones, three crustal fault zones and more than three basement fault zones and mafic and ultrabasic rock zone, granites zone, volcanic zone and contact metamorphic zone. All these zones are the favorable migration point to area and gathering area to the toxic and harmful gas. It should be highly mentioned on the construction of the tunnels.