Abstract: The morphology characteristics of male and cosexual flowers of Acer truncatum were described in details. It was found that there were two types, type S and type D, of flora in Acer truncatum, and type D was accounting for about 40% through four years continuous observation. The flora sex showed dynamic change that it can be transformed into male flowers from cosexual flowers completely. So the off year of Acer truncatum was not the result of flowering decreased, but the change of flora sex, and there may be a new mechanism of Acer truncatum blossom to be revealed.