Along with its rapid economy expanding, China is also facing the increasingly serious problem of cultural eutrophication, especially in aquatic ecosystems of lakes and reservoirs. Water bloom is the worst result of lake or reservoir eutrophication and sometimes leads them to death. To assess and forecast water bloom by studying the extent of eutrophication is crucial to the assurance of people’s drinking water and thus it is of great importance. With a case study of Lake Chaohu, the fifth largest lake in China, as viewed from the whole aquatic ecosystem, this paper synthetically considered TP, T, Chla and DO as a whole, which were the outstanding factors in eutrophication and water bloom of Lake Chaohu, to construct a water bloom cusp model. Using this model, the process that eutrophication brought on water bloom in the lake was simulated. According to the result of data fitting, monthly data of the above four water quality parameters, obtained from a certain sampling site from 2000 to 2003, fitted characteristics of cusp model. The model was tested to be accurate for its relative error was around 10%. What is more convincible, according to the catastrophe discriminant of the cusp model, it could be judged that a discontinuous jump of the aquatic ecosystem had occurred in July, 2003 in Lake Chaohu, the conclusion was consistent with t he fact that water bloom have arisen in August, 2003 in the lake. The case study of Lake Chaohu, on the base of data fitting, deduced that some water quality parameters fitted characteristics of cusp model in the process that eutrophication brought on water bloom. The research indicates that water bloom cusp model could reflect the actual state of water bloom caused by eutrophication，describing the dynamic response mode between different factors in the aquatic ecosystem and further revealing the outbreak mechanism of water bloom in the lake. This paper suggests that using catastrophe models to fit water quality data of a time serial would be a constructive approach to forecast and judge the outbreak of water bloom in lakes and reservoirs. In addition, by constructing and studying such catastrophe models, lake or reservoir managers would be able to simulate the effects of different protecting and curing projects and further supply the scientific warrant for the optimization of these projects as well. The outbreak of water bloom is the results of mutual exchange reactions of many inducements and long-term accumulative action of nourishments. As the only system theory studying how gradual change causes catastrophe, catastrophe theory satisfied the mathematical logic of the water bloom and provided us with a feasible solution for the numerical simulation of water bloom. This paper put forward a new idea for the study of the water bloom, which has never reported neither at home nor abroad so far.