Nitrogen deposition is one of the hot topics of global climate changes. In this study, effects of simulated nitrogen deposition with three different levels on the seedling growth and allelopathic potential of exotic invasion plant Spartina alterniflora were evaluated in laboratory. The results showed that both low and high concentration of nitrogen deposition significantly increased the biomass accumulation of the above-ground and below-ground parts of S. alterniflora. Its above-ground biomass under the two nitrogen deposition treatments was increased by 161.15% and 120.20%, respectively. And the below-ground biomass was increased by 128.43% and 71.06%, respectively. The leaf area and leaf weight were also increased by nitrogen deposition treatment, which would promote S. alterniflora to enhance the ability of resource capture and utilization. The water extracts of the roots and leaves of S. alterniflora under different nitrogen deposition treatments inhibited the seed germination rate and root length of Lactuca sativa. At the same concentrations of water extracts, the synthetic allelopathic effects of S. alterniflora under the low nitrogen deposition treatment were always stronger than those under high nitrogen deposition treatment and no nitrogen application. The synthetic allelopathic effects of leaves and roots reached the maximum when their water extract concentrations were 0.02 g?mL~(-1) and 0.04 g.mL~(-1) respectively under the low nitrogen deposition treatment. The above-mentioned measurement records of S. alterniflora under the high nitrogen deposition were not altogether superior to those under low nitrogen deposition, which may suggest that S. alterniflora growing under the high nitrogen deposition level would not be in nitrogen limited condition. As global nitrogen deposition elevation is a gradual process, ecological control and management of S. alterniflora will be more urgent under present nitrogen deposition condition.