Water surface spectra, which can work irradiance ratio just beneath water surface R(0^-)out, is obtained by ASD FieldSpec Hand-Held Spectro radiometer. When the in-situ data is collected, a certain special angle has been adopted. After analyzing the relationship between spectral characteristics of R (0^-)and chlorophyll-a concentration, the results showed that the strongest correlations appeared at 762 nm,727 nm and 496 nm, the values separately were 0. 85,0. 84 and-0. 80. The model analysis with single band and band ratio revealed that the quadratic models, of which independent variables were R(0^-)₇₆₂ .R (0^-)₇₆₂ R(0^-)₄₉₆,R(0^-)₇₂₇/R(0^-)₄₉₆.,were better than others. RZ separately reach 0. 923,0.919,0.916; std. error of the estimate S separately were 0. 012,0. 013,0. 013;F separately were 101. 241,96. 576,92.925. Make use of the 10 surplus samples to estimate three quadratic models and carry on the accuracy and error margin examinations. Then find that the quadratic model with the independent variable R(0^-)₇₆₂/R(0^-)₄₉₆was the best, and to a certain extent had some functions onchlorophyll-a concentration estimation in lake Taihu in summer. In addition, try to apply differential calculus to R(0^-)to estimate chlorophyll-a concentration, and can not acquire the better result.