There is a significant fluctuation in the light field under water caused by surface wave of water, to which the photosynthesis of phytoplankton in water responses. As a result, the phenomenon is attracting more and more attentions. Based on the data of light field under water in Meiliang Bay of Taihu Lake, the variation of light field under water with depth was calculated and the fluctuant characteristics of light field under water were analyzed. The results show that significant fluctuation in light field under water is resulted from surface wave of water. The maximal fluctuation depth in aptitude is approximately smaller than 20 cm, which is much smaller than that in ocean. The decrease of the fluctuation varies with depth is exponential. The larger is the diffuse attenuation coefficient, the faster is the attenuation of magnitude of the light field under water. The fluctuation will almost disappear when the depth is larger than 30 cm. In addition, the effect of boat on the measured light field under water was investigated. If the direct sunlight isn't blocked by the boat the larger proportion of the skylight in the incident light, the more relative errors just beneath the surface of water caused by the shelter of boat will be.