Abstract: Located in the key area in the west of the Longmu Cuo-Shuanghu suture, the Maoershan Formation is characterized by the special rock association, metamorphism and deformation, and hence is the key to understandiang the western tectonic evolution of the suture. There exist many questions concerning the composition and formation age due to the lack of sufficient data. The authors hold that the Maoershan Formation, which was influenced by the amphibolite phase metamorphism, is a suite of Devonian clastic rocks and carbonate rocks intercalated with basic volcanic rocks, as evidenced by the 1∶250000 Chagdo Kangri Sheet geological investigation, and its metamorphic basic volcanic rocks have the geochemical characteristics of ophiolite. The recent systematic study indicates that the Maoershan Formation is composed of tectonic slices which formed in different ages and different tectonic settings and suffered from various degrees of metamorphism. Disintegrating the Maoershan Formation is the prerequisite for understanding the material composition and tectonic evolution of this area. On the basis of field observation and measured profile, combined with the study of petrology, geochemistry and isotopic geochronology, the authors primarily divided the Maoershan Formation into four geological units: Late Paleozoic ophiolite, Early Paleozoic ophiolite, Early Carboniferous Xianshuihe Formation and Middle Triassic Lashun Formation. The basis for this division and the material composition, metamorphic characteristics, formation ages and tectonic settings of these units were summarized in this paper. This study provides new data for understanding the western material composition of the suture and the overall tectonic evolution of the Longmu Co-Shuanghu-Lancang River suture.