Abstract: A model of photosynthetically available radiation (PAR) transfer in atmosphere-water system is put forward to calculate the spatial and temporal variation of PAR in the water of Taihu Lake by using meteorological data and measured suspensions. The effects of aerosol, ozone, water vapor and carbon dioxide in the atmosphere as well as the multiscattering and absorption of water surface wave, yellow substance, suspended particles and phytoplanktons in the water are taken into consideration. Finally, the primary productivity of Taihu Lake is calculated with bio-optical model. It is shown that the results may basically reflect the variation tendency of primary productivity with water depth.