Abstract: The Cretaceous Chijinbao Formation has developed a suite of black shales with high organic content， indicating significant potential for oil and gas exploration in the Beishan area of Gansu and Mongolia. Further research into the sedimentary paleoenvironment of these black shales is necessary to enhance our understanding. Based on the MGD1 Cretaceous core data in Zhagegaonao basin， samples were collected at equal intervals. The geochemical characteristics of black shale have been studied by X-ray fluorescence spectrometry， X-ray diffraction, and inductively coupled plasma mass spectrometry. The paleoclimate， paleosalinity， paleowater depth, and oxidation-reduction conditions of black shale formation were reconstructed with geochemical data and verified by palynological data. The black shales are mainly argillaceous shale. The provenance is predominantly feldspar rock，and they are near-source sediments，which have not undergone sedimentary recycling and have not been affected by later hydrothermal action. The paleoenvironment can be analyzed with its geochemical data. The comprehensive analysis of environmentally sensitive elements， such as Sr，Ba，Cu，B，V，Ni，Co，shows that the lower section of black shale in well MGD1 was deposited in the dry-hot，saline-water and reductive environment，and the paleowater depth rose with oscillation. The middle section of black shale was deposited in warm，humid，brackish water and reductive environment，and the paleowater depth kept high oscillation. The upper section of black shale was deposited in a dry-hot，saline-water and reductive environment，and the paleowater depth decreased slightly. The accumulation of organic matter in the black shale in well MGD1 is the result of the combined action of paleoclimate，paleosalinity，paleowater depth, and oxidation-reduction conditions. 

Key words: Beishan area, Cretaceous, black shale, geochemistry, paleoenvironment, Zhagegaonao basin