The southeastern margin of Tibetan Plateau (TP) is characterized by long-wavelength,low-gradient topography with three nearly parallel largest rivers (Salween River, Mekong River, and Jinsha River) in Asian flowing out of the high plateau across this region. The formation mechanism and evolution process for the large scale landscape and drainage of the SE marginal TP remains elusive. Distributed in the tributary of Mekong upstream (29. 56°~29. 72°N and 98. 01°~98. 20°E), 5 samples are collected from Dongdashan pluton in Mangkang-Zuogong region for apatite U-Th/He dating in this study. Our new data shows that the study area experienced a cooling event at the time span 23 ~15Ma, with the cooling rate dropped from 40℃/Ma to 28℃/Ma or even lower in 5Ma, which is similar to the inversion modeling results. Moreover, most ages concentrate at around 18Ma except the sample TL09-12 which is older than 20Ma, and the age-elevation relationship plot exhibits less obvious correlation. Available low temperature thermochronology data in this region and adjacent area indicate that : the timing,for the formation of high elevation,low relief landscape of southeast and east Tibetan Plateau,is diachronity. So,it is unsuitable for discussing and calculating the regional initial uplift as one unit ; the landscape of three river regions is a non-steady transitional one,and the difference of exhumation rates suggests the geomorphic evolution of the Mekong watershed is in the period of unsteady. Furthermore, river erosion plays a dominant role in shaping landscape of this region: (1) The erosion rate in the trunk is much higher than tributary ; (2) The high erosion rate and topographic relief is consistence with intensive river incision ; and (3) The weak topographic relief is determined by the relative low erosion rate.