Biological invasions can have strong negative impacts on native biodiversity and ecosystem functions. Water level and nitrogen deposition are important factors that can affect plant invasions in wetlands. Hydrocotyle vulgaris of the Apiaceae family is a perennial herb introduced to China that can be established and multiplied rapidly in a variety of habitats. Although it is considered highly invasive, few studies have tested the effects of habitat and nitrogen deposition on invasion by H. vulgaris. We designed a greenhouse experiment to test the following hypotheses: nitrogen deposition promotes invasion by H. vulgaris in both flooded and unflooded habitats; and invasion by H. vulgaris affects the growth of native plants in both flooded and unflooded habitats. Four perennial, herbaceous species were selected to measure the competitive effect of H. vulgaris in wetland communities:Pontederia cordata, Acorus calamus, Iris wilsonii, and Myriophyllum aquaticum. These are among the most common species that co-exist with H. vulgaris in the XiXi National Wetlands Park. We assessed two levels of invasion by H. vulgaris (present or absent) with two levels of flooding (water levels of 0 cm [not flooded] and 15 cm [flooded]) and two levels of nitrogen deposition (0 and 15 g N/m² as NH₄NO₃). We measured the stem length, leaf area, numbers of leaves and nodes, and relative dominance of H. vulgaris; and the biomass and diversity of the other four plant species. Compared to the non-flooded treatment, the total mass, leaf mass, numbers of leaves and nodes, and relative dominance of H. vulgaris were significantly lower in the flooded treatment. Nitrogen deposition significantly increased the numbers of leaves and nodes in the unflooded treatment. Total mass of the other plant species combined and of P. cordata and M. aquaticum individually were significantly greater in flooded than in unflooded treatments. Neither nitrogen deposition nor the presence of H. vulgaris affected the biomass or diversity of the other four plant species. These results suggest that invasion by H. vulgaris might not exert significant adverse effects on native wetland plant communities in the short term and that flooding might decrease invasion by H. vulgaris. Control of the water level might also be a helpful measure to control H. vulgaris invasion in wetlands.