Abstract: The sedimentary environment of Permian Haersuhai Formation in Yagan area of Ejin Banner was analyzed and the evidence for evaluation of oil/gas geologic conditions of Carboniferous-Permian was provided. The sedimentary environment was studied on the basis of lithology, paleontology and sedimentary structure, in combination with the content and ratios of trace elements as well as geochemical parameters of saturated hydrocarbon by gas chromatography. The study shows that there was a large-scale regression in Yingen-Ejin Banner basin in late Middle Permian period. Marine deposits of the Upper Permian Haersuhai Formation are mainly developed in Ejin Banner-Yangan-Hangwula area. Sediments consist of pyroclastic rock and clastic rock intercalated with a small amount of carbonate rock. It is concluded that the Permian Haersuhai Formation in Yagan area was formed in an offshore-epeiric sea- continental/oceanic alternate environment characterized by brackish water.