Abstract: Ambrosia trifida L. is a monoecism plant with unisexual flowers, allergic diseases were induced by which pollens. The primary research of this paper concentrates mainly on the differentiation of the male inflorescence. Using microscope and electron scanning microscope to observe the male differentiation process of Ambrosia trifida L.. The differentiation process was divided in 3 periods that include 9 stages: vegetative growth phase(undifferentiated stage), differentiation phase of the primordium of the male inflorescence(initial period of differentiated growth point,initial period of differentiated male inflorescence,initial period of the primordium of the male inflorescence, middle stage of the primordium of the male inflorescence, telophase of the primordium of the male inflorescence), the differentiation phase of lateral inflorescence (initial period of the primordium of the lateral inflorescence, middle stage of the primordium of the lateral inflorescence, telophase of the primordium of the lateral inflorescence), male flower differentiated stage. The male inflorescence of Ambrosia trifida L. begin differentiating in seedling stage,and the plant start to differentiate when it has a pair of leaves. The lateral inflorescence began to differentiate when plant had 4-6 pairs of leaves.