There exists a phenomenon of different objects with a similar spectrum in the multispectral satellite images in Taihu Lake,which is caused by an influence of suspended matter and algae. So, it is easy to classify the water into submerged vegetations area bymistake. In this report, we firstly retrieved water transparency from Landsat TM image acquired on 15 July 2002. And then, thefollowing two types of decision trees were developed on the basis of the two vegetation index of RVI and NDVI: (a) atransparency-assisted decision tree, (b) a non-transparency-assisted decision tree. Finally, the lake was divided into the followingthree types: (a) the water area, (b) the submerged vegetation-dominated hydrophytes area, (c) the floating vegetation-dominatedhydrophytes area. The classification approach with transparency-assisted decision tree is a good choice to have a classification ofhydrophytes in Taihu Lake. We applied this approach into the Landsat TM image acquired on 15 July 2002. The classification resultsshowed that the submerged vegetation-dominated hydrophytes and the floating vegetation-dominated hydrophytes covered an area ofabout 407.6 km² and 82.2 km², respectively.