Abstract: In order to study the toxicity of herbicide pyraclonil to aquatic organisms and its risk to water environment, the acute toxicity of pyracllonil against three aquatic organisms such as Scenedesmus obliquus, Daphnia magna, and Brachydonio rerio were investigated under laboratory conditions using the alga growth inhibition, immobilization test and static test recommended by OECD, and its primary ecological risk was assessed based on TOP-RICE model. The results showed that the 72-hour median inhibitory concentration (72 h-EC₅₀) of pyraclonil on the proliferation of S. obliquus cells was 1.44 × 10⁻² mg/L, and the 48-hour median inhibitory concentration (48 h-EC₅₀) on the activity of D. magna was 15.09 mg/L, and the 96-h medium lethal concentration (96 h-LC₅₀) on B. rerio was 23.05 mg/L. According to the classification standard of toxicity grades in "Chemical Pesticide Environmental Safety Evaluation Experiment Guidelines", the corresponding toxicity grades were high toxicity, low toxicity, and low toxicity respectively. In aquatic ecosystems, groups with an acute toxicity risk quotient (RQ) value less than 1 for pyraclonil to vertebrates and invertebrates accounted for more than 60% of all simulated scenarios, and more than 60% of groups had an RQ value greater than 1 for primary producers. The results indicated that pyraclonil is relatively safe for D. magna and B. rerio, and the risk to the vertebrates and the invertebrates in the aquatic ecosystem is acceptable, but its toxicity to S. obliquus is high, and the risk to primary producers is unacceptable. Therefore, the pyraclonil should be avoided from entering the water body around the paddy fields in the production process. The risk of pyraclonil to the aquatic ecosystem can be reduced by decreasing the amount of herbicide used and adjusting the application period reasonably. It is recommended to use pyraclonil at the early tillering stage of early rice or the late tillering stage of late rice.