Allocation of light in the aquatic ecosystems determined the ecosystem function and type. Based on the measured data of optical properties and the concentrations of water substances in Lake Taihu during November to December, 2007, the optical parameters (diffuse attenuation coefficient, the average cosine) and impact factors (the proportion of absorption) were studied. The results showed that: the irradiation showed a single peak distribution and the max value was at 583nm wavelength; the dominant degree of CDOM and Non-pigment particles could be divided into weak, strong, stronger according to the K_d; the average value of K_d (PAR) and depth of the euphotic zone is 4.61±1.54m^-1 and 1.11±0.35m, respectively; solar energy was mainly distributed in the cyan, green and yellow wavelength range, which was about 60 percent of the total energy, it just about 30 percent in blue and red wavelength. This spectral structure favored the growth of Microcystis aeruginosa and Scenedesmus obliquus.