Carex breviculmis R.Br. is an ideal herbaceous plant for lawns, featuring strong drought tolerance and excellent landscaping applications. Melatonin is a hormone commonly found in nature that helps plants resist stress and adversity. However, the mechanism of melatonin to increase drought tolerance in plants remains to be investigated. In this study, four concentrations of melatonin (0, 100, 200, 300 mu mol/L) were utilized to explore the root growth index, osmotic regulatory substance content, membrane lipid peroxidation level, and the antioxidant system during drought stress. The outcomes demonstrated that, under normal water potential, there was no discernible difference between melatonin administration and no melatonin treatment, while the application of melatonin can improve the functional traits of root dry weight, total root length, surface area, forks, and volume under drought stress. Furthermore, on the 20th day of drought stress, the most notable elevations in soluble protein, sugar, free proline concentration, and POD activity, coupled with the most significant reductions in MDA concentration, were observed under 100 mu mol/L melatonin treatment, in comparison to the control group. Conclusively, the finding suggested that melatonin can effectively reduce the damage caused by drought stress. Specifically, 100 mu mol/L was the best value of melatonin to improve the drought resistance of C. breviculmis. Thus, our study demonstrates the positive role of melatonin in mitigating drought stress in plant species and provides a reference for promoting drought adaptation in sedge in semi-arid and arid regions.