Lacustrine black shales are widely developed within the Late Cretaceous Nenjiang Formation (k₂n) of the Songliao Basin. These black shales provide an ideal natural laboratory for reconstructing the paleoclimate and paleoenvironment during the Late Cretaceous in Northeast China and have great significance in the exploration for potential oil and gas resources. According to the systematic analysis of organic carbon isotopes, elemental geochemistry and clay mineral composition, the organic matter (OM) source, paleoenvironment, paleoclimate, paleoproductivity and marine transgression event of the Nen 1 member (k₂n¹) were investigated, and the influence of the paleoclimate and marine transgression event on OM accumulation in lacustrine black shales was revealed. Black shales in the lower unit are characterized by a high OM abundance (max. 4.19 wt%, avg. 3.02 wt%), I-II₁ kerogen (with abundant lamalginite) and low maturity. The element ratios (Fe/Mn, Mg/Ca and Sr/Cu), paleoweathering indexes (CIA and PIA), δ¹³C_org values and clay mineral composition together indicate a warm paleoclimate in the Late Cretaceous. The humid paleoclimate promoted the flourishing of lacustrine microalgae and surrounding terrestrial vascular plants and enhanced the lake level, conducive to favorable sources and preservation conditions for OM accumulation. Based on paleontological and radioisotope dating, the global paleotemperature rise in the Late Santonian stage probably led to the marine transgression event. It can be explained by the high lake level and sea-level conditions under the global humid climate background that resulted in the Songliao paleolake connecting with the paleo-Pacific Ocean. Alkaline and saline seawater incursion into the paleolake increased the salinity of the lacustrine water column and further aggravated the anoxic conditions of the lake, and abundant marine nutrient inputs promoted the primary bioproductivity and contributed to the accumulation of OM. The discovery of the OM accumulation mechanism under the influence of the warm-humid paleoclimate and intermittent marine transgression will add critical geological constraints on further studies of geological events in the deposition of late Cretaceous black shale.