The light quantum yield of algae particles is one of the important parameters characterizing the algae response to the environmental factors. Based on the water primary productivity and underwater irradiance at the head of the jetty as a station of the Taihu Ecological Network of the Academia Sinica the light quantum yields of algal were calculated for four observational times. Results suggest that the yields increased basically in a linear way from the just below surface down to 30 cm but further downward the yield increases were slowed down and between 60 and 80 cm the yields were level off, where the maxima were 0.12 mol (O2) /mol (photon), figures that were about 10 times those just below the surface. In spring above 60 cm in Meiliang Bay light was not a constraint of algal growing but under 60cm light was the limiting factor to photosynthesis due to light attenuation.