Abstract: The paper studies the sedimentary environment of the shales in the Lower Silurian Longmaxi formation of the complicated structure area in the southwestern Sichuan basin, and provides guidance for shale gas exploration and development in the area. Based on the systematic sampling of shales in Longmaxi formation on Niba Mountain section in Yijing county, geochemical studies are carried out. The results show that the average value of Chemical Index of Alteration（CIA）in the study area is 68, which is slightly lower than that of the shales in the world (CIA=70), and the average Th/U ratio in the study area is 3.73, which is close to the average value 3.80 of the continental crust, indicating that the provenance suffered from weak-medium weathering. Relative high content of hydrolysable and ferrum and relative low content of sulfophilic elements show that the sedimentary water was shore-shallow sea environment. The redox indexes such as δU, U/Th, V/Cr and Ni/Co indicate a weak oxidation environment of the sedimentary water, and an even stronger oxidation environment at the late deposition stage of Longmaxi formation. The paleoclimatic indicator Sr /Cu indicates an arid climate during the deposition of the provenance and paleosalinity indicators Sr/Ba and B/Ga indicate Longmaxi formation is brackish marine deposition. The TOC in the study area is relatively low and well correlates with the indicators of weathering, redox, paleosalinity and paleoclimate, which indicates that the TOC is controlled by sedimentary environment. Longmaxi formation in the study area is a set of source rocks of deep water—shallow water shelf facies, and the lower Longmaxi formation is of good quality and the upper section is relatively poor due to the influence of sedimentary environment and tectonic uplift.