This study focused on the reproductive biology of Aesculus chinensis,such as flowering phenology,flower characteristics,floral dynamics,pollen viability,stigma receptivity,pollenovule ratio (P/O),and outcrossing index (OCI).The results showed that:(1)A.chinensis was in early flowering stage from April 22,in full flowering stage from May 5 and in final flowering stage from May 14 in Xi’an.All of the flowering stages lasted about 30 d.Flowering stage of single flower lasted about 5～8 d.(2)Male flowers have degraded ovaries.There were significant differences between male and perfect flowers in flower morphology,size and physiology.The pollen viability of male flowers was far greater than that of the perfect ones.The pollen viability of male flowers maintained the highest (75.69%) during the flowering day and was the best pollination time.The stigma receptivity of perfect flowers lasted about 8～9 d,the stigma receptivity was strongest 3 d after flowering.(3)P/O and OCI results showed that the breeding system was outcrossing with pollinators.A.chinensis is a kind of andromonoecy plant.