Abstract: In this study,Scendesmusobliqaus,daphnia magna and Ctenopharyngodon idellus were selected to study the acute toxicity of Paclobutrazol to different categories of aquatic organism. The EC₅₀-96hr to Scenedesmus obliqaus is 20. 62mg/l and the LC₅₀-48hr to Daphnia magna is 21. 86 mg/ 1. The LC₅₀-120hr in static bio-test is 16. 33mg/l, while in flowing-water tests, LC₅₀-96hr is 14. 36mg/l and the LC₅₀-120hr is 9.13mg/l. According to the pesticide toxicity evaluation standard in laboratory, the toxicity of Paclobutrazol to the aquatic organisms is low. The result of preliminary safety e-valuation in field also demonstrates that Padobutrezol is low toxic. Therefore,it can be concluded that the pesticide is less dangrous to aquatic organisms. However, its chronic toxicity and long term accumulation effect are needed to further study.