Abstract: Adding quaternary ammonium compounds (QACs) as algal removal agents is an effective emergency response means for the problem of algal blooms caused by eutrophication,in freshwater lakes, and their dosage and environmental toxicity have received widespread attention. To clarify the effective concentration and safety of a new QAC algal removal agent, didecyl methylpropyl ammonium iodide (DMPAI), the toxic effects of it on planktonic algae, mainly Microcystis aeruginosa, Daphnia magna, and zebrafish embryos were determined at different concentrations. The results showed that DMPAI at the concentration of 0.5-2 mg/L could reduce 46.77% of the chlorophyll a in the water within 24 h, and reduce the number of Microcystis aeruginosa to 34% of its original level. At this concentration level, it did not affect the survival and reproduction of Daphnia magna, but retarded the growth of Daphnia magna, and the mortality rate of the zebrafish embryos was positively proportional to the concentration of DMPAI, with the maximum mortality rate of 19.00%. There were also the phenomena such as hatching in advance and heartbeat slowing, but it did not significantly affect the body length changes of juvenile fish.