The work aims to study the effect of different concentrations of betaine and melatonin on the preservation of postharvest longan (Dimocarpus longan) fruits during cold storage, so as to provide theoretical reference of the new preservation technology of longan fruits. The 'Chuliang' longan fresh fruits were treated with 0, 5, 10 and 15 mmol/L betaine combined with 300 mmol/L melatonin respectively, and stored at 4 ℃ and 85% relative humidity. The changes of the weight loss rate, soluble solids (TSS), titratable acid (TA), ascorbic acid (VC), free proline, malondialdehyde (MDA), and activities of catalase (CAT), peroxidase (POD), superoxide dismutase (SOD) were investigated. Compared with the 300 mmol/L melatonin control group, the composite preservative could reduce the loss of water, TSS, TA and VC. Meanwhile, betaine combined with melatonin manifested higher SOD and CAT activities (P < 0.05), and maintained lower levels of the MDA content and POD activity. Among them, the 300 mmol/L melatonin with 10 mmol/L betaine has the best preservation effect on longan fruits.