Abstract: Abstract: Phosphorus is one of the important limiting nutrient elements of net primary productivity of terrestrial freshwater ecosystem, and the characteristics of land runoff are an important factor affecting its loss. Therefore, it is of great theoretical and practical value to identify the key source of phosphorus loss under different runoff characteristics. In order to clarify the critical source areas of phosphorus loss in different runoff characteristics, the small watershed of Shixia in Miyun County, northeast of Beijing is taken as the study area. SCS-CN (soil conservation service - curve number) model with infiltration excess runoff mechanism and VSA - CN (variable source area - curve number) model with saturation excess runoff mechanism are used to identify the features of runoff at different rainfall types. PI (phosphorus index) model is applied to identify critical source areas of phosphorus loss in this watershed based on different runoff generation mechanisms. The results show that: 1) For infiltration excess runoff, there is high runoff in relatively humid region in the southern and eastern parts of Shixia small watershed because of the mid rainfall depth and high rainfall intensity, as well as in the arable lands in the northeastern and western parts of this region. The runoff yields in these areas are between 15 and 30 mm, which accounts for about 14.2% of the total area. There is low runoff (runoff yield less than 15 mm) for the forest land in the northern area, whose area accounts for about 85.8%. 2) For saturation excess runoff, there are 2 rainfall types. No runoff can be observed because of the rainfall type of low rainfall depth and low rainfall intensity; and the low runoff area of less than 15 mm accounts for about 99.9%. For the type of the high rainfall depth and mid rainfall intensity, the runoff yield can reach the highest level around the Miyun Reservoir watershed which takes up 14.6% of the total area with runoff yield over 30 mm; there are 17.7% of the total area with runoff yield between 15 and 30 mm and 67.7% of the total area with runoff yield less than 15 mm. 3) The degree of soil erosion in this region is weaker whereas the runoff yield is a bit higher in some areas with a higher risk of runoff. 4) For the type of infiltration excess runoff, there is a risk of phosphorus loss which accounts for about 1.4% of the total area in the southern part of the small watershed of Shixia. For the type of saturation excess runoff, there is a risk of phosphorus loss in the northern part and southern of the Shixia small watershed which accounts for about 2.3% of total area, so there is a higher risk of phosphorus loss under the type of saturation excess runoff than that under the type of infiltration excess runoff. In general, the risk of phosphorus loss in the small watershed of Shixia is lower, and the southern part of the study area close to the Miyun Reservoir is the key area for phosphorus loss and more attention should be paid for control and management.