The Qiangtang Basin is the largest Mesozoic marine sedimentary basin on the Tibetan Plateau. It is rich in source rocks and evaporites, and has become one of the most important places for petroleum exploration in China. The study of sedimentary facies of the Upper Triassic Bagong Formation in Tumen-Baqing area can not only provide evidence for the evolution of the Tethyan Ocean but also provide theoretical basis and practical guidance for the exploration of hydrocarbon and potassium resources in the Qiangtang Basin. An analysis of petrological characteristics, paleontology and sedimentary structures shows that the lagoon facies, tidal flat facies, delta facies and ten kinds of sub-facies are mainly developed in the study area. During the Bagong period of the Late Triassic, the study area was in a continental margin sea sedimentary environment under the extensional background from the north (Baqing-Suoxian) to the south (Ban-Nu suture zone).