| This paper takes the canopy interception and the influence of the canopy on natural rainfall characteristic as the research objects, tries to reveal the regulation and mechanism of the forest vegetation makes to erosive energy of rainfall, and the change mechanism of the canopy influence of the natural raindrop. Through the contrast test of the rainfall feature , rainfall energy and raindrops distribution inner and external of the forest and through the analysis on the distribution characteristics of the raindrop spectrum, to reveal the interception effect of the protection forest on improving soil structure. The objective is to provide an theoretical basis of the Effective Space Disposition and Stable Stand Structure of the Protection Forest in Beijing Mountain Area, and to provide an technical support to the construction of eco-forest engineering in any other regions which have the similar situation.According to research content and object, this paper takes the Protection Forest around Miyun Reservoir and the forest of Jiufeng Forest Park as two separate research objects, carries on the following experiments: canopy interception: though the canopy interception experiments of the protection forest around Miyun reservoir, discussed on the rainfall redistribution of the rainfall interception, throughfall, stem flow and so on in Pinus tabulaeformis and Robina pseudoacacia Plantation; rainfall feature and rainfall energy: based on the observation of the LNM in Jiufeng Forest Park and Combined with past rainfall feature to analyze the influence of the canopy on the raindrop Dynamic Characteristics, and comparison of the size, rate, Kinetic Energy and characteristics of the raindrop inner and external to research the mechanism of content in rainfall. The main conclusions this paper comes to are as follows:(1)The canopy interception is the Power Function with the rainfall in Pinus tabulaeformis and Robina pseudoacacia Plantation. Pinus tabulaeformis Plantation: Ic = 0.5785P0.5905 R2=0.7817 Robina pseudoacacia Plantation: Ic = 0.1384P0.8992 R2=0.8088(2)When the small rainfall intensity, all the rainfall is absorbed by the canopy or evaporated, so it can't have rainfall under the canopy. When the canopy is not reach the saturated water content, if the rainfall increase then the canopy interception increase. Sometimes the canopy interception is negative because the wind.(3)The raindrop size is increase along with the increase of the rainfall out forest, but D50 may decrease by high intensity rainfall. Most diameter of raindrop is small than 1mm. The diameter is rarely bigger than 3mm but has a strong contribution to the rainfall intensity. The diameter level distribution has the continuity from small to big, the amplitude is 0.15~2.6mm and its quantity increases along with the diameter level reduces. The diameter distribution doesn't have the continuity in forest and it may bigger than 3.0mm. The diameter under the canopy is increases but effective density of the raindrop is reduces. Taking the rainfall intensity as the main factor, the diameter amplitude is 0.25~0.5mm under canopy. This phenomenon showed the nature raindrops distribution is influenced by the canopy.(4)If the rainfall type is different, it can be realize by changing the different size raindrop proportion of the function. The raindrop quality and kinetic energy of the rainfall intensity of 0.34mm/h is bigger than the rainfall intensity of 0.23mm/h. The most important cause for that is the increase of the bigger raindrop (diameter>1.0mm), the amount of the total mass is 58% and 52%, and the amount of the total kinetic energy is 87.7% and 93.5%. the experiment in P.tabulaeformis plantation shows the canopy is obviously increased the kinetic energy in forest ,it is 3.8~5.2 times than out forest. The raindrop splash has a very tremendous influence to the soil erosion. The Calculation Results of the kinetic energy shows natural rainfall kinetic energy is small. The kinetic energy is increase along with the rainfall intensity increases. The rainfall kinetic energy along with the intensity change presents a smooth tendency out the forest, but the process line presents the big denticle fluctuation shape in the forest. The kinetic energy in the forest is bigger than out.The kinetic energy increases obviously in the forest when the intensity increases .Especially when the intensity is small(≤0.75mm/h).the kinetic energy in the forest is obviously bigger the out the forest. The kinetic energy in the forest is also bigger than out. when the intensity is bigger than 2.02mm/h. But if it is at the middle of the intensity the kinetic energy change tendency is not obviously.
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