The irradiance reflectance just beneath the water surface is closely related to the absorption coefficient, scattering coefficient and solar zenith angle. However, the effect of solar zenith angle on the irradiance reflectance just beneath the water surface is not clear in Lake Taihu. The influence factors of irradiance reflectance just beneath the water surface in Lake Taihu were analyzed based on the optical parameters and numerical simulates using the data of total 27 sampling sites measured in May, 2010. The results show that while solar zenith angle is in the range of 0°-25ånd 70°-89°, it has little influence on the growth of irradiance reflectance just beneath the water surface which is less than 10%. While solar zenith angle is in the range of 30°-70°, the growth of irradiance reflectance is 13.13%-52.19%. The correlation of theoretical results and measured data show that such numerical study is correct for Lake Taihu. Therefore, full consideration should be given to the effect of solar zenith angle when studying on the optical properties of water and the driving mechanism of the light to aquatic ecosystems.