Relationship of external morphology, internal anatomical structure of flower bud, and meteorological factors during flower sex differentiation of Xanthoceras sorbifolium Bunge was studied by external morphological observation, paraffin section and meteorological data observation. Results showed that: (1) The difference of male and female flower distribution rates was extremely significant between apical and lateral inflorescences in X. sorbifolium Bunge. (2) There was no difference in the morphological structure between the two kinds of flowers at the early stage of sex differentiation. Stamen development included anther wall development and microsporocyte stage (20 d), and microsporogenesis stage (11 d), corresponding to morphological changes from buds expanding to buds exposing and inflorescence elongating rapidly. The pistil development included ovule development (30 d) and megasporogenesis (11 d). The change of external morphological characteristics appeared as inflorescence elongating and bud enlarging to sepal cracking. (3) The difference between male and female flowers appeared when male and female gametophytes were formed. Megaspore mother cell division stopped during the tetrad stage in male flowers. The corresponding external appearance of the buds was green, with a transverse diameter of 2.05-4.54 mm and a longitudinal diameter of 2.99-5.32 mm. The ≥K (biological zero point) effective accumulated temperature was 230.6 ℃. The abnormal development of female flowers stamen occurred 4 days later than male flowers pistil. Mononuclear pollen grains showed vacuolization decline during mitosis stage, with flower buds transverse diameter of 12.25-18.3 mm and longitudinal diameter of 8.3-10.98 mm. The ≥K (biological zero point) effective accumulated temperature was 264 ℃. The study indicated that external morphology of flower bud development, combining with meteorological factor data, could apparently reflect changes of internal structure, and could serve as indicator for sex differentiation stage. The present study showed that the two kinds of flowers of X. sorbifolium Bunge have both pistils and stamens, and the sex differentiation is caused by the different developmental degree of pistils and stamens in the late development period. Therefore, it is suggested that, functionally, it is more accurate to define the two kinds of flowers as “female functional flower” and “male functional flower”, respectively.