Abstract: In western Sichuan Province, a low-grade metamorphic volcanic-sedimentary rock series has been discovered in Dege-Batang area on the bank of the Jinsha River. This rock series contains abundant fossils, including fossils of bivalvia, brachia, cephalopoda and Hexacoralla. Due to its complex structure, scientists have been holding diverse opinions on the tectonic forming conditions and stratification of this rock series. According to a study of paleontologic fossils in this rock series and a comparison between this rock series and rocks of the adjacent areas, the authors agree with most scholars′ opinions and define its stratigraphic period as Late-Triassic. The rock series is divided into 3 formations, namely Damadong Formation, Qugasi Formation and Tumugou Formation, in which the Damadong Formation is a stratigraphic unit newly defined by the authors.