Phosphorus export via non-point source plays a vital role in the eutrophication of water body. Identification of phospho - rus export critical source areas could help mitigate and control non-point source phosphorus export. The critical source area assessment method based on the modified phosphorus index was established, considering topographical index. Xiyuan watershed located in Yangtze-Huaihe transition region was selected as a case study area. The results show that critical source area of phosphorus export only occupies 8% of the total catchment area, and mainly distributed in the cropland close to the stream in lower reaches.