The fall armyworm (FAW), Spodoptera frugiperda (Smith), is one of the main global pests of food crops, and is on the "List of Class I Crop Diseases and Pests" in China. There are two sympatric strains, one which prefers to feed on corn and sorghum (the corn strain; CS), and another that prefers to feed on rice and pasture grasses (the rice strain; RS). Two molecular markers, the COⅠ and Tpi genes, were used to analyze the population genetic characteristics of samples collected from different parts of China. Individuals from both host strains were detected. The existence of different host strains, and the phenotypic differentiation between these, increases the complexity and difficulty of monitoring and controlling this pest. This paper summarizes progress in research on phenotypic differentiation, host adaptability, behavior, physiology, biochemistry, resistance, and other aspects, between the two FAW host strains, and the mechanism by which host strains form, both domestically and internationally. Furthermore, we discuss the significance of studying the population differentiation and formation of host strains in China. The purpose of this paper is to further enhance the prevention and control of the FAW in China, thereby preventing significant crop damage and economic loss.