Abstract: Taking the shale from the first member of Qingshankou Formation in SYY-3 well of Sanzhao sag in Songliao Basin as an example, the paper studies the petrological characteristics, including sedimentary structure, organic matter abundance and mineral composition. The results show that the shale develops massive structure and laminated structure are, with TOC of 0.50%-6.82% (averagely 3.16%) and S₁+S₂ of 2.01×10^-3-49.41×10^-3 (averagely 23.02×10^-3), generally belonging to good-best source rock. The mineral composition is mainly felsic mineral, followed by clay, with low content of carbonate and pyrite. Based on the petrological characteristics, the first member of Qingshankou Formation can be divided into 9 types of lithofacies by the classification criteria of organic matter abundance-sedimentary structure-mineral composition, among which the laminated felsic shale with high TOC has good hydrocarbon generation potential, serving as the most favorable lithofacies for shale oil exploration in Sanzhao sag.