The dark mudstones in finegrained sediments are one of the most favorable carriers for the depositional environment analysis using geochemical indicators. The Middle Permian mudstones are well developed in the Yili area of ChinaKazakhstan border. Determining the sedimentary environment during mudstone formation is critical to whether it can be the source rock of hydrocarbon and its potential to generate oil and gas. Based on the analysis of the major elements, trace elements and rare earth elements of dark mudstone, the elements which are sensitive to the depositional environment were selected in this study. The relationship between the content with its ratios of trace elements and sedimentary medium environment was analyzed. The sedimentary environment characteristics, such as climatic conditions, salinity, redox conditions of the lake basin during Middle Permian in Yili area were discussed. Our results indicate that the values of Sr/Ba, B/Ga and w(B) reflect a brackishsaltwater sedimentary environment in the Yili area during Middle Permian, with the paleosalinity of the lake gradually increasing from the early Xiaoshansayi period to the late Tiemulike period. The data of &U, Th/U and V/V+Ni）reveal that the Middle Permian of the Yili area developed under an anaerobic environment, in which water was weakly stratified with H2S gradually occurring at its bottom. Based on the standardized distribution of Middle Permian mudstone by North America shale, it can be speculated that the paleoclimatic environment was dry climate with weak chemical weathering and calm water. Comprehensive analysis above suggests that the Yili area was in a dry, hypoxia saltwater environment during Middle Permian, with the reducibility and salinity of water in sedimentary environment gradually increasing from Early Permian to Late Permian.