In this study, we performed outdoor pot experiments by use of Phytolacca americana and Dolichos lablab treated by a batch of combinations of nitrogen and phosphorus fertilization to study their competitive effects. The applied combination eight sets were low nitrogen, high nitrogen, low phosphorus, high phosphorus, low nitrogen and low phosphorus, low nitrogen and high phosphorus, high nitrogen and low phosphorus, high nitrogen and high phosphorus. The results showed that in single species of D. lablab and the mixed species with P. americana, phosphorus fertilizer and mixed fertilizer significantly increased the biomass and total biomass accumulation of D. lablab leaves. The plant height, root biomass and total biomass of P.americana were significantly increased by high nitrogen and high phosphorus treatments in both single species and mixed species of P.americana. Under the condition of high nitrogen and high phosphorus fertility, the phosphorus content in leaves, roots and roots of P.americana was significantly higher than that of the control. High nitrogen and phosphorus fertility treatment resulted that the phosphorus content in leaves, root nitrogen and root phosphorus of P.americana were significantly lower than those of the control. The leaf nitrogen content and root nitrogen content of D. lablab were the highest under the high nitrogen and phosphorus fertility treatment when it was single species or mixed with P.americana. The leaf nitrogen content was 67.17, 66.62, and the root nitrogen content was 62.07, 66.31 mg/g, respectively. With mixxing with D. lablab, the relative yield of Phytolacca americana was less than 1.0 under all fertilization treatments, and D. lablab had obvious inhibition on interspecific competition of Phytolacca americana.