Corn smut is a disease caused by Basidiomycetous fungus Ustilago maydis. This pathogen is a dimorphic fungus that needs to complete its sexual reproduction in living corn. We reviewed recent research reports of this disease, we divided the parasitization course of U. maydis into 7 stages in this paper, including the formation of pathogenic dikaryotic hyphae, attaching to the surface of host plant, penetrating the host epidermis, weakening host defense response, prolifering mycelium in host plant, inducing tumor in host tissue and the formation of chlamydospore. We also reviewed key genes involved in each stage and elaborate their function during pathogenesis. We present the sophisticated parasitic strategy of U. maydis in the process to achieve its sexual reproduction. The division of U. maydis parasitization course in this review will help understanding the interaction mechanisms between the pathogen and host plant, and provide new ideas for the prevention and control of such diseases.