In order to find out the flower sex, mating system and flower bud differentiation process of Acer truncatum Bunge, we studied the flower bud differentiation process and flowering process of A. truncatum by paraffin section and continuous anatomical observation. The results showed that: (1) there were two kinds of flower sex: functional female flower and functional male flower. The anthers of functional female flower did not crack, and functional female flower only had female function. There were two types of functional male flowers: type Ⅰ and type Ⅱ. The anther of both two types of functional male flowers could produce mature pollen and crack. The functional male flowers only had male function. (2) A. truncatum was a rare tree species of duodichogamy, and there were two mating systems: protandrous and protogynous types. The order of floret opening of protandrous type is functional male flowers→functional female flowers→functional flower male flowers, and the order of floret opening of protogynous type is functional female flowers→functional male flowers→functional male flowers. (3) The morphological differentiation of flower bud began in early July, the differentiation of inflorescence and floret began in early August, the differentiation of stamen and pistil began in late August, and the development of female gametophyte was later than that of male gametophyte. (4) The key period of flower sex differentiation of A. truncatum is from late March to early April in the following year.