To determine the ecological water demand of the Beichuan River Basin, the linear trend method, Mann-Kendall test method, and Spearman test method were employed to analyze the trend of runoff change in the Beichuan River Basin during 1959-2013, and the Pettitt test, sliding t-test, sliding F-test, and cumulative departure method were utilized to comprehensively diagnose the runoff breakpoint. The ecological water demand in the river channel was calculated based on the outer envelope of ecological flow processes obtained with the Tennant method and improved equal frequency dynamic calculation method, and the ecological water demand outside the river channel was calculated with the SWAT model. The results demonstrate that the SWAT model is applicable in the study area, and the model performance meets the criterion of “good”. Precipitation and evapotranspiration simulated with the SWAT model show increasing trends, while water yield shows a decreasing trend. The runoff at Qiaotou Hydrological Station in the Beichuan River Basin decreased at 0.104 m³/s per year during the research period, and the runoff breakpoint occurred in 1990. The ecological water demand of the Beichuan River Basin was 295 million m³ in the river channel, and ranged from 995 million to 1 085 million m³ outside the river channel, making the total ecological water demand of 1 290 million to 1 380 million m³.