Abstract: The fruiting trees of grape were used as the experiment materials, and the effects of melatonin on grape fruit quality and sucrose metabolism were studied by spraying melatonin solutions with concentrations of 0,50,100,150 μmol·L^-1 and 200 μmol·L^-1 in a field experiment.The aim of this study was to screen out the best melatonin concentration that could improve the fruit quality of grape, and in order to provide reference for the application of melatonin in grape production.The results showed that with the increasing of melatonin concentration, the shape index of grape fruit did not change significantly, but the appearance quality such as single fruit weight and the intrinsic quality such as solid-acid ratio and vitamin C increased first and then decreased.When the melatonin concentration was 150 μmol·L^-1,the appearance quality such as single fruit weight and the intrinsic quality such as solid-acid ratio and vitamin C were the best.Melatonin treatment increased the contents of glucose and fructose in grape fruit, but did not change the ratio of glucose to fructose.Melatonin had no significant effect on sucrose content in grape fruit.After spraying melatonin, the activities of decomposition enzymes related to sucrose metabolism（soluble acid invertase, soluble neutral invertase, and sucrose synthase decomposition direction） in grape fruit were higher than those of sucrose synthase synthesis direction and sucrose phosphate synthetase, making the reaction direction towards sucrose decomposition, and accumulating glucose and fructose in large quantities.In summary, melatonin can improve the appearance quality and internal quality of grape fruit, promote sucrose catabolism, and further improve the quality of grape fruit.