Abstract: To investigate the accumulation and elimination rules of avermectin in perch (Lateolabrax japonicus) muscle during actual breeding process, we determined the median lethal concentration in 96 h as 4 ng·mL^-1 based on a toxicity test, and exposed the perch to 4 ng·mL^-1 of avermectin for 72 h, and then transfered the solution into seawater. Thus, we develop a method by high performance liquid chromatography-tandem mass spectrometry to determine avermectin in perch muscle. The results show that the avermectin concentration in perch muscle gradually increases with passing medicated bath time, and reaches the peak of 8.767 ·kg^-1 at the end of the medicated bath. Later, the avermectin in perch muscle gradually decreases and reduces below the detection limit at 528^th hour.