In order to discover formation causes of short-chain steranes in coal-measure source rock of the research area, 62 samples of Triassic Xujiahe Formation in Sichuan Basin were selected for various geochemical experimental analyses. The results show that there are abnormally low pristane/phytane (Pr/Ph) and high abundance C₂₁₊ long-chain isoprenoid alkanes and dinosteranes in both humic coal and coal-measure mudstone in the study area. The difference is that the relative abundance of short-chain steranes in humic coal is higher than that in coal-measure mudstone. The relative content of short-chain steranes has no correlation with maturity parameters, but is positively correlated with “Drimaneane/Tricyclic-terpane” and “Methyl-chrysene/Chrysene,” and positively correlated with HI (hydrogen index) of source rocks. To sum up, the coal-bearing strata of the Triassic Xujiahe Formation in Sichuan Basin have experienced the influence of transgression events. Thermal effect is not the main factor leading to the formation of short-chain steranes in the study area. Transgression makes the Triassic Xujiahe Formation coal-measure source rocks in Sichuan Basin have high abundance of short-chain steranes, and the difference of bacterial activities results in the difference of short-chain steranes in coal and coal-measure mudstone.