Abstract: By using gas chromatography and gas chromatography-mass spectrometry, the paper studies the biomarker compound characteristics of Beipiao Formation source rocks from LLD1 well at the depth of 1527.91-1722.78 m (bottom unreached) in Niuyingzi sag of western Liaoning, and analyzes the sedimentary environment, source and thermal evaluation degree of organic matter. The n-alkane relative parameters of source rock samples indirectly indicate that the maturity of source rocks is low, mostly in immature-low mature stage. The organic input is mainly terrestrial source, with some lower organisms. The Pr/Ph ratio is 0.94-5.01, averagely 3.40, with the characteristics of organic matter in typical coal measure strata, formed in oxidizing environment. The C₂₄TeT/C₂₆TT-(C₂₀+C₂₁)TT/(C₂₃+C₂₄)TT value is generally high, indicating the source of higher plants. The C₂₄-tetracyclic diterpene/C₂₆ tricyclic terpane ratio is 1.15-5.28, averagely 2.76, reflecting that the terrestrial higher plants contribute a lot. The gammacerane index of 0-0.04, averagely 0.02, show a freshwater sedimentary environment. The sterane in source rock samples, showing an inverse L-shape with dominant C₂₉ sterane in the m/z217 gas chromatogram, indicates that the higher plant input is absolutely dominant. From the above study, it is considered that the Beipiao Formation source rocks in Niuyingzi sag were formed in the oxidizing environment of freshwater, with the characteristics of organic matter in typical coal measure strata. The organic matter is derived from terrestrial higher plants, which has oil-gas exploration prospect.