Abstract: The degradation occurred to the crude oil in the migration process from central depression to the margin of Songliao Basin, thus forming the oil sand reservoir in the western slope, where the extracts of oil sand are non-liquid with high density and viscosity. In Taobao-Xiaotaipingshan-Xibeigou-Tumuji area along the migration direction, for the oil sand extracts in Upper Cretaceous Yaojia Formation, the content of saturated aromatic hydrocarbons decreases with the depth shallowing, the content of unsaturated asphaltene increases, the carbon isotopes of whole oil and saturated hydrocarbon become heavier. The parameters above present an opposite variation trend in the single-well samples of Xiaotaipingshan area. Based on GC-MS analysis, the degradation levels of oil sand extracts are Wenger 2-8, the n-alkanes and isoprenoid alkanes in saturated hydrocarbons of most samples are missing, hopanes are partially degraded, and 25-norhopanes are detected in Xiaotaipingshan area. With the decreasing of depth in Taobao-Tumuji area, the biodegradation levels increase gradually. Biomarkers parameters present regular variations, such as tricyclic terpane/pentacyclic triterpane, C₂₇diasteranes/regular sterane, dibenzothiophene/phenanthrene, and etc. In Xiaotaipingshan area, the mass fractions of 25-norhopanes and corresponding hopanes as well as their ratios present that the biodegradation levels increase with the increasing of depth, the degradation resistance of hopanes increase with the carbon number increasing, and 22R homohopanes have greater biodegradation resistance than 22S homohopanes. at the western slope of Songliao Basin, the degradation laws of oil sand extracts are as below: in the migration process from a deep anaerobic environment to a surface aerobic environment, the degradation level of oil sand extracts gradually increases with the increasing of depth and water saturation in the single-well sample.