Melatonin is a new plant growth regulator that can dynamically regulate the plant systemic defense against fungal diseases through specific signaling networks. This paper summarizes the research progress of melatonin in enhancing plant resistance to fungal diseases from three aspects, namely the biosynthesis and metabolic processes of melatonin in plants, its dose-effect as a growth regulator in alleviating plant fungal diseases, and its molecular mechanism as a signaling substance regulating plant immune responses, and future research directions are prospected. Melatonin biosynthesis in plants starts with tryptophan, and completes the synthesis process through four enzymatic reactions; the key steps of its synthesis occur in the cytoplasm, chloroplasts, and endoplasmic reticula, expecially chloroplasts. Melatonin metabolic pathways mainly include hydroxylation, oxidative decomposition, and methylation degradation, with the metabolic process mainly occurring in the cytoplasm, and its metabolites jointly constitute the metabolic defense system of plants against biotic and abiotic stresses. As a plant growth regulator, the control effect of melatonin on plant fungal diseases is concentration-dependent, and its regulatory mechanisms differ among different types of fungal pathogens. In general, melatonin enhances plant immune response ability mainly through four pathways: 1) activating the antioxidant defense system to maintain redox homeostasis; 2) cooperating with phytohormones such as salicylic acid and jasmonic acid to construct a disease resistance defense network; 3) inducing the expression of resistance genes by activating the mitogen-activated protein kinase (MAPK) cascade signaling pathway; 4) regulating the synthesis of secondary metabolites to enhance the cell wall defense barrier. In conclusion, melatonin can enhance plant disease resistance by regulating the plants- own defense system. Future research should focus on scientific questions, such as the functional differentiation of key enzymes in the melatonin synthesis pathway and the evolutionary patterns of receptor proteins in non-model crops, the interaction mechanism between melatonin and novel hormones under multiple stress induction in natural environments, technical bottlenecks in field applications of melatonin (such as application stability, optimal dosage and application methods, long-term effects on soil microbial communities), and the precise localization of defense genes or key nodes directly regulated downstream of its signal transduction pathway. Combining approaches such as molecular biology, genetics, and synthetic biology to promote the precise application of melatonin in the green prevention and control of plant fungal diseases.