The geochemical characteristics of natural gas play an important part in the research of accumulation mechanism. The characteristics of gas component and its carbon isotope in Kelasu tectonic zone were discussed, and the maturity of the natural gas was analyzed. It is indicated that natural gas in this large gas field is dominated by hydrocarbon, and the content of methane accounts for more than 90%. In general, natural gas in the overpressured large gas field from Kelasu tectonic zone is typical dry gas with high carbon isotope of methane and ethane. The research result has important designative meaning for accumulation of overpressure large gas field.