Based on the remote sensing hyper-spectral reflectance measured in the Lake Kun-cheng in May, 2009 and concurrent chlorophyll-a (Chl-a) concentration assayed in the lab, a three-band model(semi-analytical method), was applied to estimate the concentration of chlorophyll-a to monitor the water quality of the lake Kun-cheng located in Chang-shu city, Jiangsu Province. Compared to the estimating results from the band ratio method based on normalized spectrum and the first derivative method, the results show that the estimating results from the three-band model was more exactly. The three-band model can improve the estimation accuracy for Chl-a by analyzing the inherent optical properties of the chlorophyll-a, the total suspended matter, the CDOM and the pure water and optimizing the choice of the three bands.