A dynamic model associated with daily air temperature and daily global radiant flux model was developed to simulate the growth dynamics of Phragmites austmlis. The biomass of shoots and rhizomes were analyzed with respect to various radiant flux, temperature curve and accumulated temperature under the condition of abundant nutrient. The variation of the biomass was related to the total accumulated temperature of early growing season by stored material of rhizomes and of photosynthetically growing season, and the ratio of the respective accumulated temperature, and the daily flux rate of solar radiation. Analysis of annual flux showed that the respiration of aboveground and belowground plant organs was an unavoidable component and changed the energy budget of utilizing photosysthesized materials, caused the variation of the biomass.