Abstract: Based on the interactions relationship among nutrients, producers and consumers in eutrophic water bodies, a mini model of a class of nutrient-phytoplankton was constructed and the qualitative and stability analyses were carried out. The sufficient conditions under which the number of positive equilibrium is 0, 1 or 2, respectively, were obtained and the corresponding stability and saddle-node bifurcation were discussed. Numerical simulations were also given to verify the mathematical results. That the model has 0 or 1 positive equilibrium indicates that the phytoplankton such as algae are extinct or persistent under certain conditions; That the model has two positive equilibria indicates that the phytoplankton tends to different equilibrium in different initial state.