To understand flower bud morphological differentiation and related physiological biochemistry changes of Syzygium cumini, provide the basis for controlling the quantity of flower and cultivation management, we observed the morphological characteristics of flower buds by frozen section and scanning method, and the related physiology indexes were determined by anthrone colorimetry, coomassie brilliant blue G250 and enzymelinked immunosorbent in this experiment. Results indicated that: (1) the flower bud morphological differentiation of S. cumini could be divided into seven periods: inflorescence primordium differentiation period, floret primordium differentiation period, sepal primordium differentiation period, petal primordium differentiation period, stamen primordium differentiation period, pistil primordium differentiation period and stamen and pistil formation period. (2) During the flower bud morphological differentiation of S. cumini, the length of flower bud showed a “fastslow” growth trend and the width showed a “fastslowfast” growth trend. (3) As for the changes of nutrient and endogenous hormone contents in flower bud, total soluble sugar decreased firstly and then increased, the content of protein rose significantly. (4) During the flower bud morphological differentiation of S. cumini, the content of GA₃ was extremely low, while ABA, IAA and ZT rose, the ratios of ABA/GA₃ and ZT/GA₃ increased while ABA/IAA decreased. In conclusion, during the seven periods of flower bud morphological differentiation of S. cumini, a massive accumulation of carbohydrate and protein, low concentration of GA₃, high concentrations of ZT, ABA, IAA, ABA/GA₃ and ZT/GA₃ contribute to the flower bud morphological differentiation.