Lake Erhai is suffering from eutrophication and frequent cyanobacteria blooms. Based on the Aquatox model, this study simulated the routine water quality dynamics and the annul succession of algae, established a set of parameters applicable for the Lake Erhai ecosystem, and validated the simulation results. Though there is a little difference in the trend of TP with measured data, water-temperature, TN and Chl. a concentration were all well coincided with measured data. In addition, this paper tries to discuss the response of eutrophic state(TN, TP, Chl. a) to nutrient input(nitrogen and phosphorus). The results show that Lake Erhai ecosystem is sensitive to the nutrient input. Comparing to the TP regulation, TN regulation has a more significant effect to Chl. a concentration especially when it was reduction control. They both can increase the Chl. a concentration in a small degreed. There is an enhanced coordinated effect in Chl. a decreases when regulating TN and TP at the same time. Furthermore, if the TN concentration is kept at or under the threshold of 0.34 mg/L, which means to cut the TN input more than 43.3% as before, the eutrophication process can well be restrained with a change in the dominant algal specie and a sharp fall of Chl. a concentration.