To reveal the difference in the rates of underwater decomposition between living plants and litters in Poyang Lake wetland, this study was conducted involving a microcosm experiment with two predominant species-Carex cinerascens and Phalaris arundinacea-using the litter bag method. Fresh and dried plant samples were prepared to simulate the decomposition processes of living plants and natural litters, respectively. The results showed that:(1) The decomposition process of both plants (C. cinerascens and P. arundinacea), lasting 180 days, had a quick primary stage and a slower later stage. (2) The decomposition rates of both litter samples were significantly correlated with their C/N ratios, but similar correlation was not found in the living samples. (3) The underwater decomposition rate was related with the species; the living P. arundinacea decomposed faster than its litters, while living C. cinerascens decomposed slower than its litters. (4) At the end of the experiment, 34%-43% of the living plants remained undecomposed. We speculated that aquatic erosion and foraging by animals had important contribution to the disappearance of large plants after the lowering of water levels in the Poyang Lake wetland.