Abstract: The simulation experiment was carried out in Taihu Laboratory for the lake ecosystem research from May 8 to June 24 in 1999.In order to find the distribution of light intensity and variation of primary productivity due to the increase of total suspended matter caused by water flow,the attenuation law of underwater light intensity,the relationship between the diffuse attenuation coefficient,the light euphotic depth and the total suspended matter,and the effects of light on primary productivity were studied.The results show that the light intensity attenuated as the depth increased,showing the good negative index correlation under not only the immobile condition but also weak and strong water flow.From immobile condition to weak and strong water flow,the diffuse attenuation coefficient increases from 1 m^-1 to 2 m^-1 and 4 m^-1,and euphotic depth decreases from 4 m to 2 m and 1 m.There are a good linear correlation between diffuse attenuation coefficient and total suspended matter as well as euphotic depth.Under immobile condition,the maximum primary productivity was recorded between 0.4 m and 0.6 m due to light inhibition at surface water shined by strong light.Under other circumstance,the maximum primary productivity was recorded between 0 m to 0.2 m due to the increase of the total suspended matter caused by water flow.