Abstract: Based on effects of a bio-retention system on rainwater, a simulation test on rainwater runoff was carried out. With the sequential extraction method, contents of exchangeable inorganic phosphorus(Ex-P), aluminum phosphate(Al-P) and iron phosphorus(Fe-P) in the filtration medium or soil of the bioretention system at different depth (0, 5, 15 and 35 cm) were determined and TP in the effluent of bio-retention system was also monitored. Results show that the adsorption of Ex-P, Al-P and Fe-P by the soil medium decreased with the depth, layer by layer from the top to the bottom. During the process of adsorption, Ex-P gradually transformed into Al-P and Fe-P, while, Al-P did into Fe-P. As a result, Ex-P decreased in concentration while Al-P and Fe-P increased with the depth. About 90% of the TP in the runoff rainwater was intercepted and removed by the system. The phosphorus, not absorbed by plants, remained in the system, being a part of the dynamic TP balance of the water-soil-root-biome system.