The downwelling diffuse attenuation coefficient（Kd） indicates the information of water quality.It is also key parameter of bio-optical model and attracting more and more attentions.Based on the data of light field under water in Meiliang bay of Taihu lake

the changes of Kd with water depth is analyzed.The results show that

Kd takes on significant fluctuation with the depth above 60cm

which is caused by the surface waves of water.But the fluctuation with depth weakens

and approximately levels off

even showing the characteristic of asymptotic light fields below 60cm

because the effect of scattering is gradually significant.In the region of 0—60cm

there is significant linear correlation between the degree of fluctuation of downwelling diffuse attenuation coefficient and the absorption of water

namely

the absorption of water strengthens the effect of surface wave on the light fields in water.The Kd of shortwave takes on evident fluctuation with the depth compared with that of longwave

because The detritus and CDOM in water significantly absorbs the radiation of shortwave

lowers diffuse light

and intensifies the effect of focusing or defocusing of surface waves.