Abstract: Environment change alters hydrological rhythms and affects utilization of water resources for a river basin. The Lancang-Mekong River is a transboundary river in Southeast Asia originated in the Southwestern China. It is of significance to scientifically quantifying runoff change attribution of the Langcang River for supporting water benefits share between countries which the Lancang-Mekong River runs across. Based on the recorded runoff data series from 1961 to 2020 gauged at the Yunjinghong hydrometric station on the Langcang River, the variability characteristics of runoff series and the impact of climate change and human activities on runoff change as well as its seasonal distribution were investigated by using statistical methods and hydrological simulation methods. The results show that: ① The measured annual runoff at Yunjinghong station showed a significant decrease trend from 1961 to 2020, with an increase in runoff during the period from January to May and a decrease in runoff in period from June to December. The abrupt change of the measured annual runoff series was occurred in 2008. ② The RCCC-WBM model performs well for monthly natural discharge simulation in the baseline period of 1961—2008 with relative error of less than 1.0%. The simulated natural annual runoff in the human-disturbance period of 2009—2020 is about 371.8 mm, which is 6.8% less than that in baseline period. The reduction is induced by climate change. ③ Reservoirs construction and operation is the utmost human activity in the Langcang River. Human activities are the main reason of runoff decrease in the reservoir impoundment period of 2009—2013, while climate change is principal driver of runoff reduction in in the reservoir operation period of 2014—2020, which contribution is about 61.7% of total runoff reduction. ④ The reservoirs operation makes runoff decrease by 44.3% in flood season from June to November and increase by 134.3% in dry season from December to May for period of 2014—2020. The regulation function of reservoirs on the Langcang River for decreasing flood discharge and increasing low flow has been guaranteeing security of water supply and flood control for the middle and lower reaches of the Mekong River.