Abstract: The gas chromatography thermo-conversion mass spectrometry (GCTC-IRMS) can be used to detect the hydrogen isotopic composition of the natural gas.An experiment has been conducted first to optimize the MS ion source parameters and the GC conditions.The effects of the H₃⁺ factor,the GC flow rate and the carbon stick-coated conditioning of the thermo-conversion reactor on the analysis results have been discussed.The results prove that this isotopic measurement method has high reproductivity when compared with the numerous analysis results of the certified reference natural gas.The ana-lysis results of the hydrogen isotopic composition of some gas samples from the Sichuan Basin are pre-sented.Combined with the existing regional geological data,the hydrogen isotopic differences between the individual hydrocarbon gases prove that the gas in the shallow Jurassic reservoir in the western Sichuan Basin is coal-formed hydrocarbons from the high to over mature continental source rocks.The results also show that the hydrogen isotopic value of the methane from the eastern Sichuan Basin is higher than-135‰,heavier obviously than that from the western Sichuan Basin,indicating that the gas from the eastern Sichuan Basin is typically characterized by the marine origin.