By applying biological slow filter of continuous growth of algae,the paper carried out advanced purification experiment.The degradation mechanisms of nitrogen and phosphorus in the system was evaluated through the analyses of pollutant contents in different depth areas of biological slow filter pool.The results showed that in biological slow filter pool,the removal of total nitrogen mainly depends on the function of a combined denitrification and dephosphorization reactor.The degradation rate of total nitrogen reaches 31.5%.While the removal of total phosphorus mainly relies on both algal absorption and microbial adsorption, The degradation rate of total phosphorus reaches 47.0% Moreover, the decrease of ammonia nitrogen concentration mainly relies on volatilization and nitrification in the overlying water,the degradation rate of ammonia nitrogen concentration reaches 21.8%. There are large amount of microorganism and algae in the overlying water and filter material surface of biological slow filter pool,which have key role in purification of water quality.