Abstract: China is one of the few countries in the world which have made breakthroughs in discovery of gas hydrate in both offshore and inland.It has therefore provided good platforms for study of gas origins.In this paper, we collected the data from different layers and different occurrences of the Shenhu area, South China Sea and the permafrost region in the Qilian Mountains.Geochemical studies of hydrocarbon gas are carried out for gas origin.Our results indicate that free gas from drilling cores is of significance and effective to the identification of genetic-type and origin of gas hydrates.If the composition and isotopic characteristics of the free gas are identified with the gas hydrate, it indicates a consistent gas origin.However, if there shows difference in gas composition characteristics and that there is negative deviation of methane carbon isotope in a vertical sequence for the same hole, it means there is vertical migration.As the conclusion, free gas can provide effective information for identifying hydrate or its genetic types.