Gas geochemistry, fluid inclusion salinity, core observation and thin section analysis indicate that the Permian-Triassic Changxing-Feixianguan gas reservoir experienced chemical alteration of thermochemical sulfate reduction (TSR) since the middle Yanshanian period. The TSR alteration is mainly shown in three aspects: (1) The C_ 2 and 12C relationships to CH_4 and 13C are preferentially consumed, resulting in the gas dryness coefficient to be larger and carbon isotope heavier; (2) A great deal of water produced by TSR makes the original formation water in the gas reservoir diluted, so that the salinity of formation water is reduced correspondingly; (3) TSR and its products, CO_2 and H_2S, make the dolomite and anhydrite dissolved, which increases the porosity and then improve the physical property of reservoir.