Abstract: The formation of the Irrawaddy River system was closely associated with the growth of the Tibetan Plateau. Hindered by difficulties in approaching the steep and rugged terrain in field reconnaissance, researchers have failed to clearly understand the formation time of the Irrawaddy River. The formation history of the tributaries of its upper reach can provide important implications for understanding the formation of the modern Irrawaddy River watershed. The Longchuan River which flows through the Mangbang basin in Tengchong area is the first grade tributary of the Irrawaddy River. Studies show that a paleolake existed and the modern course Longchuan River was not formed in the Mangbang basin in Pliocene. The Longchuan River established its modern path after early Pleistocene. This interpretation is based on the channel morphology with which the Longchuan River carved its course through the middle and late Pliocene and Pleistocene volcanic rock coherently. The age of the Pleistocene volcanic rock is about 0.84-1.4 Ma in Tengchong. From the age of the Longchuan River, the authors infer that the modern Irrawaddy River system was built not earlier than 0.84 Ma. The northward propagation of the eastern Himalaya syntaxis might have supplied an important tectonic driver for the landscape evolution of Irrawaddy from late Cenozoic to the present.