Abstract: This research developed a systematic method of calculating waterdrop kinetic energy of rainfall and throughfall and explained how a forest canopy redistributes the rainfall kinetic energy and controls the throughfall kinetic energy. It was thus proposed that not all forests are suitable to functioning as water and soil conservation.For atmospheric precipitation, the number of raindrops distributes in a normal distribution, i.e. N(a,b), where, a is the average diameter of raindrops and b is the standard deviation. My research shows that there was the following relations among a,b and the precipitation intensity (P) that determines a and b,a = 1.32665 + 0.319051nP (r > 0.998);b = 0.75551 + 0.00223P(r > 0.995).However, for thoughfall, the a and b are not determined by P but by the structure of canopy. For a definite canopy ,a and b are definite. This is shown below for canopies of some tree species: Cunninghamia lanceolata: N (3.7, 0.98 ); Pinus massoniana: N (3.5, 0.97 ); Sassafras tzumu: N (4.1.1.14) :Eucalyptus exserta;N(3.9,1.09);Acacia mangium:N(3.8,1.06). For most canopies,the diameter of throughfall(in fact, it is not all throughfall but just dribbling waterdrop from leaves of canopy) is larger than the diameter of atmospheric precipitation raindrop, which will increase the dash of waterdrops to the forest land, especially in the situation of low intensity of rainfall. Therefore, in general, the forest canopy will hamper the topsoil conservation on forest land. However, forests often have underlayer vegetation which, unlike the forest canopy, actually plays an important part in soil conservation. The main function of forest in aspect of water and soil conservation is to protect the deep soil from landslide with its complex root system. When making water and soil conservation forest, we should consider a number of factors, such as precipitation characteristics in that region, the density of trees, the height of forest canopy, coexistence with other vegetation and so on. Herbs have always no negative effects on water and soil conservation, especially in keeping top soil. However, to protect the deep soil from landslide, herbs may have little function. Therefore, it had better to combine with mixed forest to strengthen the effects.