Abstract: The degradation of nicosulfuron mediated by Bacillus cereus and the toxicity of nicosulfuron and its degradation products to environmental organisms were investigated. UPLC-TQD was used to analyze the degradation products. At the same time, the toxicity of nicosulfuron and its main degradation product to Brachydanio rerio (zebrafish), Daphnia magna and Scenedsmus obliquus were determined according to “chemical pesticide environmental safety evaluation test guidelines”. The results showed that four degradation products, 2-amino-4,6-dimethoxy pyrimidine ( Ⅰ ), 2-N-(aminomethyl) sulfamoyl-N,N-dimethylnicotinamide ( Ⅱ ), 2-1-(4,6-dimethoxypyrimidin-2-yl) ureido-N,N-dimethylnicotinamide ( Ⅲ ) and N-(4,6-dimethylformamide) oxypyrimidin-2-ylcarbamoyl)-1-(methylimino) methanesulfonamide ( Ⅳ ), were detected. And 2-amino-4,6-dimethoxy pyrimidine was the main degradation product. It is speculated that the degradation of nicosulfuron mediated by B. cereus was proceeded via the C—N and C—S bond cleavage in the sulfonylurea bridge and the cleavage of the pyridine ring. The toxicity study showed that nicosulfuron and the degradation product Ⅰ had "low toxicity" to zebrafish, with the 96 h-LC₅₀ values of 16.95 mg a.i./L and > 100.00 mg a.i./L, respectively. Then the ErC₅₀ of nicosulfuron and the degradation product Ⅰ against S. obliquus at 72 h were 8.070 and 142.7 mg a.i./L, respectively, which indicated "low toxicity". The EC₅₀ of nicosulfuron and the degradation product Ⅰ against D. magna at 48 h were 9.190 and 51.95 mg a.i./L, respectively, and their toxicity levels were "medium toxicity" and "low toxicity", respectively. In a word, the toxicity of the main degradation product to the three aquatic organisms were significantly reduced compared to nicosulfuron.