Abstract: The direction of incident radiance is changed by the coarse surface of waterwhich caused lry thewind.At the same tune, the reflectance, the translnittance and the geometrical structure of the incident light beneath the surface of water is also changed.In this paper, the mean cosine of the downwelling incident light fields jixst beneath different coarse surfaces due to different wind speed in a cloud free sky is investigated by means of the ray tracing.The results show that the geometrical structure of the downwelling light fields beneath the surface of water and their trends of changed are significantly affected by wind speed.When the zwnith angle of direct sunlight is small, the percentage of diffused light in the atmosphere is higher, and the decreasing mean cosine is smallwe with the incresing wind speed.While in a bigger zenith angle (as 80°), the mean cosine increase with the wind speed, and this trend is more clear with the increasing of direct sunlight.In the condition of higher percentage of the direct sunlight, the difference of the mean cosine of different zenith angle is inversed with the wind speed.This research is helpful for inversion of the inherent optical property of the water, and it can improve the precision of estimating primary production.Moreover, this method acn reduce the computations and get quite precise results.