For many years, there have been controversies over the formation time and tectonic attributes of the volcanic rocks of the late Triassic that are widely exposed from Yunxian to Nanjian, western Yunnan. A few meters of thick dacite, dacite tuff intercalated with rhyolite and tuff were newly discovered in the original Middle Triassic Manghuai Formation in the Manwan area of Yunxian County. In this study, the zircon U- Pb LA- ICP- MS dating was carried out on D0001 andesite, and the magmatic crystallization age of 262. 5±3. 1 Ma and the inherited age of 447. 4±9. 1 Ma were obtained. The age was determined to be late Middle Permian, not the Middle Triassic as considered by predecessors. The detailed study of rock assemblage isotopic dating and geochemistry shows that the volcanic rocks belong to the Carboniferous- Permian Longdonghe Formation and are a set of intermediate- acid volcanic rock assemblages. The content of SiO2 (68. 14%~74. 62%) has weak deficit in varying degrees and falls into the dacite- rhyolite area in the TAS diagram. The trace element cobwebs are obviously enriched in Rb, Th, Ce and other large ion lithophile elements and depleted in high field strength elements such as Nb, Ta, Ti, which are characterized by volcanic island arc.It is the youngest island- arc acid volcanic record within the Lincang magmatic arc. The magma source area is the partial melting of mafic rocks in the lower crust mixed with crust source materials. It should be the product of the eastward subduction of the early and middle Permian Changning- Menglian Ocean. This discovery provides favorable petrological records and chronological constraints for the eastward subduction of the Changning- Menglian Ocean in the Sanjiang area in the southwest, and also provides favorable support for the north extension of the Carboniferous- Permian Longdonghe Formation. The distribution and scale of the Manghuai Formation needs to be reassessed.