At present, conventional monitoring frequency of water quality is relatively low, and it is difficult to objectively reflect the variation of the non -point source pollutants with rainfall -runoff process. In this circumstance, the export load of non -point source pollutants in a basin is sometimes underestimated. This paper used the synchronous monitoring data of water quality and volume in a basin to study the causal relationship between an individual rainfall event and the export phosphorus load and estab - lish relevant relation between the individual rainfall amount and export phosphorus load. The results show that they have good quantitative relationship. Making use of this kind of quantitative relationship and individual rainfall amount, the export phosphorus load in the basin can be reasonably estimated.