Abstract：Damming can significantly impact riverine mercury (Hg) transport. However, knowledge on the source and transport of Hg in rivers of high altitude drop is lacking. Temporal and spatial variations of Hg concentration along the Lancang River were monitored from July 2017 to June 2018. Results showed that Hg concentration in Lancang River showed a similar trend with annual runoff distribution; however, contrasting seasonal changes between upstream and downstream reservoirs were observed mainly due to different land uses and runoff recharge along the flow direction. The increasing trend of ratio of particulate Hg to total Hg (DHg/THg) and reservoir discharge caused the Hg flux to increase from 144 kg·a?1 to 788 kg·a?1 when focusing on changes along cascade reservoirs. Correspondingly, the Hg interception rate of cascade reservoirs from upstream to downstream gradually decreases from 88% to 65%. The results of this study provided new insights into the source and transport of Hg in cascade dams.