Abstract: Carbon dioxide gas, bursting in the deep drill hole of XCDCSZ-3-1 during the final survey of Xi′ ningChengdu Railway, is characterized by high pressure and long duration of eruption. The genesis, distribution, migration and storage characteristics of CO2 gas were studied based on a survey of regional geological background and by means of comprehensive adoption of geophysical technology, drilling, gas sampling and chemical test. The alignment principles were also studied by analyzing effect of high pressure gas on tunnel in a perspective of geology. The results indicate that high pressure CO2 gas is of inorganic and caused by thermal decomposition of basal marble, the migration channel is mainly palaeo weathered crust which mainly accumulates in the snout-like bulging structure of baserock;distribution of high pressure CO2 gas is closely related to lithology, depth, structure of baserock, lithology of overlying strata and superficial fault, route scheme optimization can be achieved by bypassing relevant geological factors.