Abstract: This study investigated the geochemical characteristics and origin of natural gas in H₂S-bearing gas reservoirs in Sichuan Basin. Results show that H₂S-bearing natural gas in Sichuan Basin generally has a dry coefficient close to 1, thus is classified as dry gas. The gas contains high contents of non-hydrogen components including CO₂, N₂ and H₂S, whose compositions vary significantly in gas reservoirs among different oil-gas fields. The sulfur isotope value of H₂S gas varies from 9.3 ‰ to 31.0 ‰ and shows a strong correlation with that of anhydrite in the same reservoirs, indicating that the H₂S components of natural gas and anhydrite in the same reservoirs share identical sulfur source, and that the former is a product of thermochemical sulfate reduction(TSR)reaction. The H₂S content has no significant relation with the hydrogen isotope value of methane. However, the CO₂ content increases with increasing H₂S content, demonstrating that CO₂ generation is closely related to the H₂S component of natural gas in reservoirs. In addition, the H₂S content has a strong relation with the carbon isotope value of ethane, possibly due to TSR reaction that preferentially consumes C₂₊heavy hydrocarbons and thus results in heavier carbon isotope values of ethane. This result once again proves that TSR reaction occurs in H₂S-bearing reservoirs.