Abstract: To address the deficiency in river segment-scale environmental risk zoning, this study constructed a refined classification method for sudden water pollution risk in river reaches. The method was applied to the North Canal River Basin in Beijing, building upon the environmental risk field assessment and a review of existing literature. The results indicated that high-risk river reaches spanned 86.0 km, accounting for 17.4% of the total; medium-risk reaches spanned 169.8 km (34.4%); and low-risk reaches spanned 237.4 km (48.1%). River reaches at high risk of secondary sudden water pollution from stationary sources were mainly located in the Changping section of Wenyu River; the Daxing section of Xinfeng River; and the Daxing and Tongzhou sections of Liangshui River. River reaches at high risk from mobile sources were primarily distributed in the Changping sections of Wenyu River, Dongsha River, and Nansha River; the Shunyi section of Xiaozhong River; the Chaoyang section of Tonghui River; the Fengtai and Daxing sections of Liangshui River; and the Daxing sections of Xinfeng River and Feng River. Finally, we recommended implementing differentiated prevention and control measures for river reaches at high risk of sudden water pollution from stationary sources, such as regular inspections of potential hazards at high-risk enterprises and enhanced protection of sensitive receptors. For river reaches at high risk of sudden water pollution from mobile sources, transportation controls should be strengthened and protective engineering measures for sensitive receptors reinforced.