In an artificial climatic cabinet, Ranalisma rostratum was subjected to three water depths which were 0 cm, 3 cm, and 6 cm, and two light intensity treatments which were 400 μmol m^-2 s^-1 and 240 μmol m^-2 s^-1. By recording the biomass, foliar morphological characteristics, and other physiological characteristics, the adaption of R. rostratum to different water depths and light intensities, and the reason why the plant is endangered were concluded. Under the same light intensity, with increasing water depth, proline content decreased; petiole length increased; leaf biomass, stem biomass, root biomass, total biomass, leaf width, leaf area, peroxidase activity, soluble protein, and soluble sugar content initially increased but subsequently decreased; and malondialdehyde content initially decreased but subsequently increased. At the same water depth, low-light intensity stress decreased leaf biomass, stem biomass, total biomass, peroxidase activity, soluble protein, soluble sugar, and proline content. It can be concluded that lack of light intensity and a change in water depth can all lead to R. rostratum's dysplasia and the plant cannot adapt well to these adverse conditions. Therefore, in the wild, sharp changes in water depth and the shade from upper plants may be vital reasons for the endangerment of R. rostratum.