The first member of Upper Cretaceous Qingshankou Formation (K₂qn₁) in Songliao Basin has considerable shale oil resource potential because of the wide development of dark mudstone and high residual hydrocarbons. The paleo-sedimentary environment directly controls the dark mudstone distribution, organic matter development, and residual hydrocarbon content. Therefore, analyzing the sedimentary features and restoring the paleo-environment of K₂qn₁ are of great significance for revealing the regularity of shale oil enrichment. Based on the core observation and logging data of well Jishanyou 1 in Qian'an area of Changling Sag, the authors analyzed the sedimentary characteristics of K₂qn₁. According to the element data obtained by natural gamma ray spectrometry logging and lithologic scanning logging, the evolution of the paleo-environment of K₂qn₁ was restored by using suitable parameters. Furthermore, the correlation of paleo-environment parameters and residual hydrocarbon content in mudstone shale was also studied. The results show that the dark mudstone of K₂qn₁ in the study area was developed in the outer Delta front to shallow lake-semi-deep lake sedimentary system. The palaeo-environment of K₂qn₁ underwent a change of water depth from shallow to deep, climate from semi-humid to humid, salinity from fresh to brackish, reduction conditions from weak to strong, and sedimentary hydrodynamics from moderate to weak. There is an obvious interface of sedimentary environment within K₂qn₁. The content of residual hydrocarbon in the mudstone is mainly controlled by paleo-climate and paleo-reduction conditions. The oil-bearing organic matter was formed in a wet and reducing environment, with strong oil-generating capability and high content of residual hydrocarbon.