The input and transfer of external phosphorus in freshwater lakes is a major factor which affects the process of a lake eutrophication. Domestic and foreign scholars conducted a lot of researches on the nutrient subtances such as phosphorus and made many important progresses. However, the researches were more focused on the release and transformation of internal phosphorus but less on the input and transfer of external phosphorus. In this paper, we analyzed the progresses of exrernal phosphorus from its sources, morphological characteristics, migration and transformation, and the impact on the algae. The input, shape and migration of external phosphorus were clear now, the main research trend in the future should focuses on the interaction bwteen the influcial factors which impact the morphology, migration and transformation and the quantitative relationship between various phosphorous forms and the growth of the algae.