Taking Polygonum hydropiper as the target plant and Carex brevicuspis as the adjacent plant, the changes of growth and ecological stoichiometry characteristics of P. hydropiper under different water levels(30 cm, 0 cm and -30 cm)and competition patterns (no competition, full competition, above-ground competition and below-ground competition) were studied through outdoor control experiments. The results showed that:the biomass accumulation of P. hydropiper was significantly affected by water level under different competition patterns, and which decreased significantly with the increase of water level. The maximum biomass was 10.84±1.52 g in the no competition pattern with the -30 cm water level. Under 30 cm and 0 cm water levels, the biomass accumulation of P. hydropiper had insignificant difference under different competition patterns. However, at -30 cm water level, the biomass accumulation of P. hydropiper under the below-ground competition pattern was significantly higher than that under the full competition pattern and the above-ground competition pattern, which indicated that the competition between P. hydropiper and C. brevicuspis was dominated by the above-ground competition under the non-stress condition. The nitrogen and phosphorus contents in leaves, stems and roots of P. hydropiper were significantly affected by water level. The nitrogen and phosphorus contents of leaves were significantly higher at 30 cm water level than those at other water levels. At -30 cm water level, the ratio of C:N and C:P was significantly higher than those at other water levels, which was 48.08±3.85 and 590.3±43.4, respectively. Compared with the control treatment, competition decreased the total nitrogen content of P. hydropiper, while the C:N ratio increased and the N:P ratio decreased, which may be due to that the competition reduced the nitrogen absorption of P. hydropiper.