A dynamic model associated with shoot density variation along water depth gradient including landward site has been developed to illustrate the growth dynamics of Phragmites australis. The environmental factors affecting the strength of tillering of vertical rhizome has been considered. The validity was confirmed by the below ground and above-ground biomass ratio and shoot height. The tendency that the growth potential of landward reed is changed dynamically by the water defficiency or nutrient condition was well represented by vertical rhizome length and land height. The growth potential curve is shifted to landward or waterward, when water depth become deeper or shallower than the water depth when the rhizome was formed, respectively. The importance of landward reed is depicted as a bufferzone of water-level fluctuation. The difference between potential biomass and observed data indicates nutrient or healthy conditions and the thickness of organic layer is closely related the difference.