This study presents an approach for the determination of tlie trophic state from field reflectance spectra in Lake Nanhu of Changchun, which was in eutrophic status. Reflectance spectra were measured from April to July in 2004, with ASD FieldSpec spectrometer. Based on in situ water sampling and reflectance measurements, algorithms of Chlorophyll-a, TN and TP have been developed. With ihe TSI_M trophic state index model, the lake trophic state are monitored and evalaated. The model eqnations as follows:TSI_M(chla)=10[2.46 + ln(Chla)/lnt2.5)] TSI_M(TN)=10[2.46 + (0.3155 ln(TN) + 4.5998)/ln(2.5)] TSI_M(TP)=10[2.46 + (0.3939 ln(TP) + 5.4415)/ln(2.5)] TSI_M(AVE)=[TSI_M(Chla) + TSI_M(TN) + TSI_M(TP)]/3 The result shows:1) TSI_M model to evaluate lake water trophic state, may obtain accurate result normally, with little time, effort and money; 2) The TSI_M trophic states of hyperspectral simulation data, compared with that of laboratory data, narrow the differences of the same point in different months and different points in one same month; 3) Lake Nanhu is in serious eutrophic state, and needs to take measures to prevent the water quality from worsening further.