Plants show phenotypic plasticity in response to changing environments via variations of morphological and ecophysiological traits, and the plasticity of plants can increase markedly the capability of survival, growth, establishment, recruitment and distribution. In the real world, terrestrial plants suffer multiple stresses when they are submerged, such as oxygen deficiency and shading due to the lower gas diffusion and light intensity in water. Light climate and dissolved oxygen availability are strikingly different in aquatic environments from that in terrestrial habitats. It has been reported in many studies that most flooding-tolerant plants adapt to flooding through high phenotypic plasticity. However, few studies have investigated the effect of both light and dissolved oxygen levels on functional response in morphological plasticity of plants under submergence condition. To reveal the effects of light (L) and dissolved oxygen (DO) in water on the morphology and growth of Alternanthera philoxeroides, one of the common invasive plant species in the Three Gorges reservoir region, an experiment was conducted and the morphological traits and biomass allocation of A. philoxeroides were analyzed in the ecological garden of Southwest University. Flooded groups and non-flooded control group were set in the experiment, A. philoxeroides were put to grow in water with light (+) or dark (-) and high (+) or low (-) levels of dissolved oxygen. Flooded groups included L+DO+, L+DO-, L-DO+ and L-DO-four treatments. The following parameters were measured before flooding: stem length, internode number, internode length, stem diameter, leaf number, leaf length, leaf width, leaf thickness, leaf angle. At the end of the experiment, we harvested the whole plant, including above-ground parts and below-ground parts, and measured the same parameters above mentioned and some new parameters, such as leaf disk area, adventitious root number, adventitious root length, shoot number and shoot length. The plants were separated into main stem, leaf, adventitious root, shoot, and under-ground parts, and cleaned by tap water. All plant parts were oven-dried for 72h at 75℃ and then weighed respectively. It was found that: (1) Inundation promoted the plastic responses of A. philoxeroides and caused the stem and leaf elongation growth. Stems and leaves became slender and erect during the flood. The elongated stems help to keep the top parts of plant above the water surface for gaseous change and photosynthesis. (2) Phenotypic plasticity of A. philoxeroides was influenced by light and dissolved oxygen in water. There were significant differences of responses to light and dissolved oxygen among various parts of A. philoxeroides. The stem elongation growth was mainly affected by dissolved oxygen level, while the leaf was mainly influenced by light intensity. Under the same illumination level, stem length, internode length and number, and stem length-to-diameter ratios were found to be greater for plants living in high dissolved oxygen environments. Whether light existed or not, the largest stem length and internode number of A. Philoxeroides were found at the high dissolved oxygen levels after flooding, 61.8% and 34.2%, respectively. In addition, foliar morphological characteristics of A. philoxeroides altered significantly when the light existed. The ratio of leaf length-to-width, specific leaf area and leaf angle of A. philoxeroides were increased by 39.65%, 28.3% and 45.9° after flooding. (3) Light and high dissolved oxygen can promote the emergence and growth of shoots and adventitious roots, respectively, and therefore, they improved the ability of survival and spread of A. philoxeroides under water. This study may help to further understand the mechanism of A. philoxeroides how to escape from the submergence. It also confirmed the importance of light and dissolved oxygen in affecting the population establishment of invasive A. philoxeroides under submergence condition.