Granulation is a physiological disorder of citrus fruits, which causes juice sac crystallization, reduction of juice, loss of sugar and organic acid, and even affects the edible value and commercial value of citrus fruits. In this paper, the physiological and molecular mechanism of citrus fruit granulation is systematically expounded from cytology, fruit quality and nutrition, hormone level, enzyme activity and molecular biology. This review lays a foundation for further research on the mechanism and control measures of citrus fruit granulation.