Triassic volcanic rocks are widespread in southern Lancang River zone of Sanjiang area. Middle-Late Triassic volcanic sequences exposed in full in Yunxian area in the north of southern Lancangjiang zone, can be divided into three formations as Middle Triassic Manghuai Formation (T2m), Late Triassic Xiaodingxi Formation (T₃x) and Late Triassic Manghuihe Formation (T₃mh) from the bottom to up. Geochemical characteristics indicate that Triassic volcanic rocks from southern Lancang River zone have dual characteristics of arc volcanic rocks and continental intraplate volcanic rocks, suggesting that their formation environment is transition-type continental margin orogenic belts. With the dating results, the main collision stage of Lancang River is inferred in Early Triassic. Middle Triassic and early Late Triassic are respectively the stress relaxation stage after collision and continued subduction stage of oceanic basin. Until the end of Late Triassic, subduction ended, Lancang River Ocean closed.