Abstract: To further research the tectonic settings of Mesozoic fault basin in Daxinganling Mountains, on the basis of field geological survey and petrogeochemical analysis, this paper systematically studies the sedimentary environment and geochemical properties of Middle Jurassic Wanbao Formation in Suolun area. The results indicate that Wanbao Formation is a set of continental sedimentary clastic rocks distributed in the intermountain fault basin, with the sedimentary environment of braided river delta facies. The Wanbao Formation is geochemically characterized by enrichment of LILEs (Th, U, LREE), Zr and Hf, and depletion of HFSEs (Nb, Ta, P, Ti), as well as Ba and Sr. The REE patterns reveal right-dipping LREE enrichment. The sediment provenance mainly comes from acid-intermediate magmatic rocks, with the characteristics of continental island arc tectonic setting, which may be related to the southward subduction and back-arc extension of Mongolia-Okhotsk Ocean.