Abstract: Based on gas chromatography and gas chromatography mass spectrometry techniques, the authors studied biomarker characteristics of the Ganquan Formation source rocks in Dahulishan area of Ejin Banner, analyzed the information and petroleum geological significance indicated by sources of organic matter, sedimentary environment and maturity as well as some other factors. Gas chromatographic peak of saturated hydrocarbons from Ganquan Formation source rocks shows the single peak distribution of the"frontal peak", with the main peak of carbon being C18，indicating that the main sources of organic matter were of marine organic substances; Pr/Ph ratios range from 0.3 to 0.6, indicating a strongly reducing sedimentary environment. Biomarker assemblages of Ganquan Formation source rocks have abundant tricyclic terpane with long side chains, contain high level of gammacerane, and show dominant distribution of C27 steranes. High content of gammacerane in organic matter suggests that organic facies source rocks were formed in a saltwater sedimentary environment. The average hopanes / steranes ratio of 0.59 implies that algae made more contribution to organic matter than bacteria, and high level of C27 steranes suggests that alga was the main source of hydrocarbon precursors. The methylphenanthrene index of Ganquan Formation, which is an effective molecular parameter for measuring thermal evolution of organic matter, ranges from 0.35 to 0.50, and the conversion of vitrinite reflectance Rc from this index is 2.00-2.09, indicating that Ganquan Formation source rocks were at the over-mature stage of thermal evolution.