Abstract: Mexel^®432, as a new biological inhibitor, can be used to treat biological filth in the pipes of cooling water circulation system in power plants. In this study, the acute toxicity of Mexel^®432 on three common marine economic organisms: Haliotis discus hannai, Mesocentrotus nudus, Charybdis japonica was studied by semi-static water experiments. The results showed that there was a significant time-effect and dose-effect relationships between the toxicity of Mexel^®432 to three organisms. The median lethal concntrations of Mexel^®432 were 8 mg/L, 11 mg/L and 186 mg/L, and the safe concentrations were 0.80 mg/L, 1.10 mg/L and 18.60 mg/L, respectively. After stress, the toxicity of Mexel^®432 to the three marine organisms was Haliotis discus hannai＞Mesocentrotus nudus＞Charybdis japonica.