Abstract: Based on analysing the petrological characteristics and fossil plants in Hongshan Formation and lower part of Sanjiaoshan Formation,south east Xiaoxinganling,the authors concluded that Hongshan Formation and lower part of Sanjiaoshan Formation are synchronous deposition in two different areas,belong to Late Permian. Because of Sanjiaoshan Formation named later,the authors propose that Sanjiaoshan Formation must be cancelled and only use the name of Hongshan Formation to represent late Permian terrigenous deposits.Pyroclastic rock of the upper part of the former Sanjiaoshan Formation is incorporated into Wudaoling Formation.It must be emphasized that several coal seams are discovered in researched area,which is a important clue to seek late Permian practical coal beds in south east Xiaoxinganling.