In order to learn the formation mechanism and developing characteristics of unknown gas in a tunnel on SichuanTibet Railway and to find a new method for study of unknown gas, the study methods for engineering geology and geochemistry are analyzed. The unknown gas, the mixed gas, is identified according to sample test, allweather monitoring data and study of the formation mechanism and developing characteristics of the unknown gas. And then, the possible migration of the unknown gas from other regions is eliminated according to the study of engineering geology, the distributions and geological structures of gas resources and coalbearing strata and rock fissure developing conditions. Finally, the laboratory test and geochemical analysis are carried out on rock sample, which demonstrate the organic and inorganic formation mechanisms, small storage and the autochthonous generation of the unknown gas. The study can provide reference for followed successful construction of the tunnel.