Abstract: Abstract: Three tributary rivers (which are Bibei River, Motan River and Heishui River) in Beibei District of Chongqing were studied in this paper. Comprehensive evaluation of five monitoring cross sections and eleven pollution factors were conducted by fuzzy mathematical comprehensive evaluation model. The evaluation result showed that the water quality of the five monitoring cross sections has clearly seasonal changes, and the surface water quality of Heishui River is the best in the whole year, with Grade Ⅰ and Ⅱwater quality standard, met the standards of water environmental quality in addition to Pianyan section was Grade Ⅲ in February. However, water quality of Motan River is the worst, even with a bad Grade Ⅴ water quality standard, so it was in serious pollution and needs to be treated. The water quality were in Grade Ⅳ water quality standard in February and Grade Ⅲ in May and Grade Ⅱ in August. The evaluation results provide a scientific basis for pollution control.