Abstract: In order to examine the variations of interspecific competitive abilities of the Oryza Sativa and its coexisted invasive weed Alternanthera Philoxeroides under nitrogen additions, the differences of biomass and fluorescence parameters between these two plant species were compared by using the de Wit instead pot experiment, as well as the impacts of nitrogen additions on them. The results showed that the maximum stem length, root length, root area and leaf area of O. sativa were significantly higher than A. philoxeroides in the mixed cultures, and the biomass and relative yield of O. sativa were also significantly higher than A. philoxeroides at the whole. But the nitrogen balance index, chlorophyll index, flavonoid index, minimum fluorescence and maximum fluorescence after dark adaptation of A. philoxeroides were all significantly higher than those of O. sativa. Nitrogen additions made the total nitrogen content of O. stiava be significantly higher than A. philoxeroides. However, nitrogen additions significantly increased the steady-state fluorescence decay rate and photochemical quenching coefficient of A. philoxeroides but decreased the steady-state fluorescence decay rate of O. stiava. In conclusion, O. stiava had the higher morphological and biomass advantages, while nitrogen additions improved the competitiveness of A. philoxeroides in photosynthetic physiology.