Based on the theory of atmospheric boundary layer and having synthetically considered various factors which may affect air turbulent intensity, we developed a numerical model for calculating the effect of waterbody on air turbuleat exchange coefficient (ATEC) and obtained the following conclusions.1. The difference of ATEC depends on the differences both on roughness and on atmospheric stability between waterbody and land. As the roughness on water surface is always less than on land surface, its effect on ATEC is stable. However, the difference of atmospheric stability over waterbody and over land has periodic variation, i.e. air over waterbody is more stable than over land in the daytime; in the nightime, the opposite holds. Thus its effect on ATEC is also of periodicity,2. In the daytime, the ATEC over waterbody is less than that over land, and there are two minimum centers. (1) near water surface; (2) at the altitude of loom over waterbody.3. In the nightime, the ATEC near water surface is still less than near the land. But in the leeward of waterbody, it is larger than over the land; especially in summer, it may be double as much,4. The profile of ATEC is increasing to the maximum from surface at first, then decreasing to zero with increasing height. The altitides where the maximum ATEC occurs are about 400-500m in the daytime and about 130-200m in the nightime.The ATEC in the daytime may be more than double as much as in the nightime.5. The maximum difference of ATEC over waterbody and over land occurs where the maximum ATEC occurs.The distribution of the difference with height is similar to the profile of ATEC.6. Over water surface, the diurnal variation of ATEC is less than over land, and the diurnal change range near water surface is very small.