[Background]It is one of the challenging issues in water resources management as influence of climate changes and human activities have intensified on the hydrological cycling of watershed. As a component of streamflow,baseflow plays a significant role in water balance analysis,optimal allocation and management of water resources,as well as the maintenance of healthy river ecosystems. However,it has rarely been explored on the impacts of climate change and human activities on the baseflow up to date. [Methods]We,therefore,used well established baseflow separation method,Chapman-Maxwell filtering,to separate the baseflow from in situ measured daily streamflow data for Chaohe watershed located in the upstream of the Miyun Reservoir from 1963 to 2015 in northern China. Mann-Kendall test and double mass curve approaches were then applied to track the change trend and change points of annual baseflow and to quantify the contributing percentage from climate change and human activities for the studied period. [Results]1) From 1963 to 2015,the baseflow in the Chaohe watershed significantly declined with the change point occurred in 1999,baseflow decreased by average annual of 0.1 mm. Before and after the change point,the annual average precipitation decreased by 16.4% ( 84.4 mm), the annual average PET increased by 7.3% ( 84.4 mm), and the annual average base flow decreased by 66.8% ( 9.2 mm) . 2) Climatically,the baseflow was significantly correlated with precipitation in a positive way ( R~2 = 0.64,P < 0.01) while negatively correlated with Penman-Monteith equation derived potential evapotranspiration ( PET) significantly ( R~2 = 0.27,P < 0.01), precipitation played a more important role than PET in baseflow variation. 3 ) Decreasing baseflow was mainly caused by human activities with maximum contribution rate of 81.10%. [Conclusions] The human activities were the main reason for the decreased baseflow from 1999-2015 in Chaohe watershed. Such activities include land use /land cover changes,massive ecological restoration,water conservancy project construction,and water withdrawal and water uses in the watershed. More specifically,the implantation of soil and water conservation measures and land conversion programs( Grain for Green) in the Chaohe watershed made significant increase in forestland and decrease in grassland and cropland, which favored more evapotranspiration and leading to the reduction of baseflow.