Euphotic depth is one of the most important variables in the study of an aquatic ecological system. This variable has direct effects on the distributing of phytoplankton and net primary production of waters. The underwater reflectance and photosynthetic available radiation (PAR) data were collected from Oct. 24 to Nov. 2, 2006 in Taihu Lake. For retrieving euthotic depth, a variety of relationship models were attempted in which in situ reflectance at a single band or band ratio was regressed onto measured euthotic depths. The eutrophication of Taihu Lake was then assessed based on the correlation of this parameter to transparency which was derived from the estimated euthotic depth. The results indicate that euphotic depth and normalized in situ reflectance are highly correlated, and the relationship models can be sued to accurately retrieve euphotic depth from a specified spectral variable; among the algorithms examined, exponential equations performed better than other relationships; and the models with the band-ratio resulted in higher retrieving accuracy than those using a single band; the assessment of the eutrophication with euphotic depth has proved to be a valuable and applicable approach. The application of this model to Taihu lake indicates that the western area of the lake was serious eutrophic, and the eastern area of the lake was mesotropher and light eutrophic.