Tight sandstone gas constitutes a strategically significant resource in the exploration of unconventional hydrocarbon systems. Current understanding of the geochemical composition and genesis of tight sandstone gas in the Daning–Jixian Block, southeastern Ordos Basin, is insufficient, which hampers a comprehensive assessment of its resource potential. This study is the first to systematically investigate the geochemical characteristics and genetic origin of high-maturity tight sandstone gas in the southeastern Ordos Basin’s Daning–Jixian Block. Gas specimens were systematically acquired from multiple stratigraphic units within the reservoir interval and subjected to compositional and carbon–hydrogen isotope analysis. Compared with other gas fields in the Ordos Basin, the Daning–Jixian Block has higher average methane concentration, and notably lower ethane and propane concentrations; its average δ13C1 and δ2H-CH4 is heavier, while δ13C2 and δ13C3 are lighter. Based on the δ13C1-δ2H-CH4 diagram, all gas samples from the block and other basin gas fields fall into the geothermal, hydrothermal and crystalline gas domain, indicating gas genesis associated with over-mature organic matter interacting with external hydrogen. Milkov genetic diagram analysis reveals that the natural gas consists of primarily early-stage kerogen-cracking gas, with a minor contribution from crude oil-derived gas originating from Carboniferous–Permian source rocks. Notably, samples from Daning–Jixian exhibit a unique δ13C1 > δ13C2 reversal, attributed to mixing effects between gas from highly mature kerogen and gas from secondary cracking of crude oil. Consequently, ethane carbon isotopes alone are insufficient for definitive genetic classification. These findings provide a new geochemical interpretation framework for analogous high-maturity tight gas reservoirs.