The Songliao Basin is a large-scale continental lacustrine basin in China. The Upper Cretaceous Nenjiang Formation developed high-quality source rocks, which were mainly formed in the first (K2n1 ) and second members of the Nenjiang Formation (K2n2 ). However, the redox environment has been debated over years. The Houjingou section, in the typical area of Songliao Basin, is selected as the study site. By measuring and analyzing organic carbon and nitrogen isotopes, trace elements and other geochemical indicators of the source rocks in this area, it is found that there are obvious differences between K2n1 and K2n2 . The higher TOC value, the negative deviation of δ13Corg value and the smaller value of δ15N in K2n2 than those in K2n1 indicate that the anoxic environment formed in K2n2 after marine transgression. Combined with trace element ratios, the paleo-productivity parameters such as Ba/Al and Ba/Ca reflect the higher productivity of K2n2, which is consistent with the change of TOC; the redox parameters such as Mo/Al and V/Cr indicate the water environment is more reductive in K2n2 deposition, which is consistent with the conclusion that the δ15N value is small indicating the formation of anoxic environment in K2n2. The redox environment of the water body during the deposition period is reconstructed. The research results can not only predict the distribution of high-quality source rocks, but also provide a reference for the study of sedimentary environment of other lacustrine shales in China, and provide a new basis for oil and gas exploration.