According to the spectral model for transferring of solar radiation in cloudless atmosphere, the solar irradiance at the surface of Taihu Lake is computed and ratio of photosynthetically available radiation (PAR) to the total radiation investigated.Also,the numerical computations are carried out by comparison with the twice 24-hour field observations;and it is shown that the computed results are the enveloping curves of the measured data.In the numerical computation,the meteorological data,such as surface pressure and wind speed,from the meteorology stations in Wuxi region are adopted;and in the treatment of reflection of water-surface.the effect of wave is taken into consideration.