Abstract: Seasonal fluctuation of water levels is commonly found in natural and constructed wetlands and the tolerance of a plant to such fluctuation is a key factor for the composition of community and distribution of wetland species. Understanding how the growth and morphology of plant species respond to different water levels and plant densities is important to help predict their productive potential under certain environments and better manage the wetlands. In this study, Hemarthria altissima was planted at four densities (1, 2, 4 or 12 plants/pot) and four water levels were imposed (Control, CK; Wet-dry alternation group, FD; Waterlogged group, LF; Flooded group, FL) to study the morphological and growth responses. The growth and morphological characteristics of H. altissima were influenced significantly by different water levels and densities. Growth was inhibited significantly in the FL group, while in the FD and LF groups, the total biomass, aboveground biomass and stem biomass of the plants were high, except in the groups planted at higher densities and there was no significant effect of densities under LF. The length of internodes and the main stems were greatest in the LF treatment, except for the group with 1 plant/pot. With increase in plant density or in the level of water stress, the total root biomass and the number of leaves and shoots decreased gradually. Moreover, with increasing flooding stress, the relative neighbor effect and its value increased gradually, and the intensity of competition decreased, but the intraspecific interaction did not shift to facilitation. H. altissima showed the ability to adopt different strategies under different water conditions, either to escape from partial submergence by further shoot elongation or to adopt a non-elongating quiescent strategy when completely submerged. Hence, H. altissima is a suitable species for planting in the regions with varying soil water availabilities. Based on the results, higher planting densities are recommended in completely flooded areas and medium densities are suitable in the partially flooded areas or in the ecotone between terrestrial and aquatic environments.