Phosphorus concentration with aluminum(Al) compound is a pretreatment method for the dissolved phosphorus analysis by 31P-NMR.In order to apply this method to concentrating phosphorus in freshwater of Lake Taihu，experiment conditions of this method were analyzed，including pH of water during Al precipitating dissolved phosphorus，amount of Al compound，volume of water and type of extractant.The results showed that the amount of Al compound needed in the phosphorus enrichment is related to not only the phosphorus concentration in freshwater，but also the volume of water.The best experiment conditions and procedures to concentrate phosphorus for 31P-NMR analysis were selecting 20 L filtration water from Lake Taihu，and adding 3 g AlCl3?6H2O in the water (keeping pH value of 6.0).After overnight，Al floc containing phosphorus was collected and then redissolved in 0.5 mol?L-1 NaOH+0.1 mol?L-1 EDTA.