In order to study the response of runoff to climatic changes, the multiple regression analysis and sensitivity analysis were used to identify the changing trends between runoff, precipitation and air temperature. Based on the long-term measurements from the mid and upper reaches of the Fenhe River, the relationship between climate factors and runoffwas established and 25 climate-change scenarios were hypothesized. The results with quantitative analysis show that climatic warming has resulted in sharp increase of evaporation, and lead to the decrease of runoff in the main stream to a certain extent. Furthermore, the dry-and-warm climatic conditions may dramatically cause the probability of droughts in this area.