Abstract: The present study aimed to explore the degradation effects and mechanisms of 24-epibrassinolide（EBR）treatments on acetamiprid, carbendazim, difenoconazole and myclobutanil in watermelons. Watermelons were used as test materials, and EBR and water treatments were set as experimental group and control group, respectively.After spraying pesticides, samples were taken at 2 h, 1 d, 3 d, 5 d, 7 d, 9 d, 15 d, and 24 d, and liquid chromatography tandem mass spectrometry was used to determine the residues contents of the above pesticides in watermelon fruits, and absorption photometry was employed to determine detoxifying enzymes activities, reactive oxygen species contents and metabolites changes. The results showed that, compared with water control group, the EBR treatment significantly reduced the contents of acetamiprid, carbendazim, pheniconazole and myclobutanil in watermelon fruits（P＜0.05）, reduced the production of malonaldehyde（MDA）, significantly reduced the production rate of superoxide anion（ O₂）, and maintained high detoxification enzymes activities including superoxide dismutase（SOD）, catalase（CAT）, peroxidase（POD）, L-phenylalanine ammonia lyase（PAL）, glutathione reductase（GR） and ascorbate peroxidase（APX）, and kept the contents of soluble proteins, reduced glutathione（GSH） and hydrogen peroxide（H₂O₂） at higher levels. Spraying EBR could increase the contents of detoxifying substances and the activities of detoxifying enzymes in watermelon fruits, and finally reduce the contents of pesticide residues in watermelon fruits.