Based on effect factors of transforming from point to area of infiltration rate and crust etc., put forward the formula of changing soil point's infiltration rate to area's rate on small watershed, which is fm=fn譙n/S譨n譩n. Here, fm is the average soil area's infiltration rate(mm/min) on behalf of different land using models and topography and landforms, fn is the average soil point's infiltration rate(mm/min) under different land using models and topography and landforms through many field tests, Sn represents areas of different land using models(hm²), S is Watershed's area(hm²), an means correct coefficient thinking the effect of crust and system mistake etc., bn is the parameter thinking the effect of soil's water content. Comparing the area's infiltration rate between transforming value and observing value, the result showed that relative error was only 2.42 percent., Preliminary gain the equations, which have high precision including single and fixed factors equations between soil area's infiltration rate and different soil control degree, rain quantity. The method is advantageous for seeking rain, runoff, infiltration's changing trend after comprehensive control on small watershed.