In view of the sharp reduction of river runoff and river base flow in the Fenhe River Basin and the serious deterioration of ecological environment in the basin, according to the measured runoff data of Lancun hydrological station, the upstream control station of the Fenhe River Basin, the applicability of digital filter method and time step method to the base-flow separation in the Fenhe River Basin was compared. The variation characteristics and main influencing factors of base flow were analyzed, and the importance of influencing factors was evaluated by using random forest algorithm. The results show that the digital filter(α=0. 925)is suitable for base-flow separation in the Fen River Basin and can objectively reflect the base flow situation in the Fen River Basin. The river base flow showed a very significant downward trend from 1956 to 2016, and eased after 2008. The mutation occurred in 1981 and 2008. The annual distribution of base flow in the upstream of Fenhe River Basin showed a typical bimodal structure. The peak flow shifted from August to March. The average base flow from 2000 to 2009 is only 10% of that from 1956 to 1959. The contribution rate of precipitation reduction to base flow reduction is only 9. 96%. When the influence of temperature is not considered, the contribution rate of human activities to base flow reduction is 83. 19%. In human activities, coal mining and overexploitation of groundwater are the main factors causing the reduction of base flow.