The Chaobai River Basin, which provides majority freshwater for Beijing City, is an important guarantee to water safety in Beijing. So it is of vital importance to assess the future change in discharge in the Chaobai River Basin under 1.5℃ and 2.0℃ global warming. In this study, the hydrological responses to climate forcing under 1.5℃ and 2.0℃ global warming in the Chaobai River Basin were studied using hydrological model SWAT driven by five general circulation models (GCMs) under three Representative Concentration Pathways (RCPs). The uncertainties of projected impacts from GCMs structure and RCPs scenarios were estimated and compared quantitatively further. The results indicated that: (1) The simulation results of SWAT model have good agreement with discharge observations in both calibration and validation period, it is feasible to apply the SWAT model to estimate the impacts of climate change. (2) The mean annual temperature and precipitation of the Chaobai River Basin will increase under 1.5℃ and 2.0℃ global warming. The mean annual discharge will increase slightly under 1.5℃ global warming and be more concentrated in summer and early autumn. When under 2.0℃ global warming, the mean annual runoff will rise by more than 30%. However projected mean proportion of monthly to annual discharge will decrease in the peak season (July and August) of water usage, which will have impact on water resource management and control. (3) Extreme monthly and annual discharge will increase dramatically under global warming, especially in 2.0℃ warming. As a result, the risk of flooding will grow and lead to more pressure on flood mitigation. (4) It is proved that there are more uncertainties in above results under 2.0℃ global warming than under 1.5℃ global warming. Uncertainties in projections of discharge are greater compared with those in precipitation projections. Under both 1.5℃ and 2.0℃ global warming, uncertainties in the projections of all indicators from GCMs structure are larger than those from RCPs scenarios.