Abstract: The model describes the dynamics of the four main algae (Cyanophyta,Chlorophyta, Cryptophyta, Bacillariophyta) and zooplanktons in West Lake on the yeary time scals. The result of the model calibration in satisfactory, and the model can reasonably respond to the changes of forcing functions in the ecosystem. The model has been used to forecast some changes in the four main algae with changes in the quantity of drawing water from Qiantang River, and in the phosphorus content of drawing water or stream water and the removal quantity of sediment. The effect of improving water quality with the measure of harnessing was evaluated.