Whether plants can synthesise and stored abundant carbohydrate in the terrestrial environment in hydrofluctuation belt after water flooding have important influence on their longterm settlement, survival and spread of the plants living in hydrofluctuation belt. Cyperus rotundus is one of the few perennial herbs that able to adapt to the habitat of hydrofluctuation belt in the Three Gorges Reservoir area. In order to study the changes of C. rotundus photosynthetic physiology and its formation mechanism in the terrestrial environment, we regularly observed the photosynthetic physiology of C. rotundus which grow in the deep water (22-27 m), shallow water (2-7 m) and nonflooded zone in hydrofluctuation belt of Three Gorges Resevoir area after water flooded. The results as follows: (1) the net photosynthetic rates (P_n) of C. rotundus growth in deep water and shallow water were significantly higher than that in nonflooded area; (2) the P_n, stomatal conductance, apparent water use efficiency, apparent light use efficiency and apparent CO₂ use efficiency increased with the extension of C. rotundus living in terrestrial environment; (3) the light saturation point of C. rotundus which grow in shallow water was significantly rose, while the light compensation point was significantly decreased. Positive selfadjustment ability and the photosynthetic compensation mechanism of C. rotundus after water flooded stress are important reasons for its adaptation to hydrofluctuation belt in the Three Gorges Reservoir area.