Algae distribute from the tropics to the poles, and are important marine primary producers. Some algae gradually form different ecotypes during their adaptation to different environmental conditions and environmental changes. In this article, the definition and research methods of algal ecotypes are summarized. Algal ecotypes should be determined according to their ecological niches, biological and genetic characteristics. Through analyzing the current situations and problems in this field, it is concluded that phenotypic traits can be used as reference factors, but not as absolute standards. A new floating ecotype is defined in Ulva (Enteromorpha) prolifera, after analyzing the environment of the Yellow Sea, genetic differences and biological characteristics of the floating population.