Phragmites australis are the typical vegetation in Zha Long National Natural Reserve. From August 10 to August 21st in 2006, we chose 60 quadrats in 15 sample sites, investigated the reed biomass, height, diameter, water nitrogen and phosphorus concentration. The results are as follow: 1) the average biomass of Phragmites australis models is 2858g•m-2 and the biomass between 15 sample sites is from 2029.0 to 4067.4 g•m-2; 2) the moisture content of Phragmites australis is from 42.95% to 64.95%. The average height of Phragmites australis is 272.47cm, and the average diameter of Phragmites australis is 0.51cm; 3) the NP weight ratio in each sample site concentrate from 9.18 to 15.79. Nitrogen is the most important limiting factor in Phragmites australis vegetation growing while phosphorus is not the limiting factor; 4) the Phragmites australis biomass is restricted by many factors especially water depth and water nitrogen concentration. But the phosphorus concentration in water has irrelativeness with Phragmites australis biomass.