Abstract: In order to explore the effects of postharvest exogenous melatonin (MT) treatment on the storage quality of spaghetti squash, squash was treated with 0.2 mmol/L MT solution to investigate the changes in fruit quality and antioxidant capacity during storage at (9±1) ℃ with relative humidity of 65%~70%. The results showed that MT treatment delayed the occurrence of decay of spaghetti squash by 40 days during storage compared with the control group. At 160 d of storage, the decay rate of MT-treated fruit was only 15.56%, which was significantly lower than that of the control fruit (34.07%, P<0.05). At the end of storage, compared with the control group, MT treatment reduced the weight loss rate and respiration rate of fruits by 29.68% and 18.02%, respectively, and delayed the degradation of hardness, brittleness and color. In addition, MT treatment increased the peak activity of superoxide dismutase in fruits by 14.18% and effectively enhanced the activities of catalase, peroxidase and ascorbate peroxidase, reduced O₂⁻· production rate, significantly inhibited the accumulation of H₂O₂ and malondialdehyde. MT treatment also significantly improved the contents of total phenolics and ascorbic acid (reduced) in fruit (P<0.05), enhanced the scavenging capacity of DPPH·, ABTS⁺· and ·OH. Comprehensive analysis indicated that postharvest MT treatment could effectively enhance the antioxidant capacity and maintain the better quality of spaghetti squash during storage.