The high content of nitrogen and phosphorus in the northern farmland drainage ditch is the main source of eutrophication in the downstream water body. In order to improve the removal effect of nitrogen and phosphorus in the drainage ditch, a pot experiment was carried out in Hetao Irrigation District of Inner Mongolia. According to the low, medium and high nitrogen and phosphorus concentrations in the drainage ditch, three plant combination modes of reed, cattail and reed + cattail were set up, and three planting densities of 15 plants/m2, 30 plants/m2 and 50 plants/m2 were set up for each mode of aquatic plants, including 30 treatments in the blank control. The effects of plant combination patterns and planting density on the purification of nitrogen and phosphorus at different drainage concentrations were studied, and the purification effects of different treatments were comprehensively evaluated based on the entropy weight TOPSIS model. The results showed that for different concentrations of nitrogen and phosphorus in water, single aquatic plants and combined aquatic plants had higher removal efficiency of TN and TP in water under different planting densities. The average removal rate of TN under the combined mode was 7.41 percentage points and 15.61 percentage points higher than that of single reed and cattail, and the average removal rate of TP was increased by 11.71 percentage points and 19.32 percentage points, respectively. In addition, the removal rate of TN and TP by aquatic plants was proportional to the planting density, compared with the planting density of 15 plants/m2 and 30 plants/m2. The average removal rates of TN were increased by 4.75 percentage points and 1.61 percentage points, respectively, and the average removal rates of TP were increased by 6.58 percentage points and 2.64 percentage points, respectively, under high planting density (50 plants/m2). At the same time, the removal rates of TN and TP in the three combined modes were positively correlated with the water concentration. The average removal rates of TN by aquatic plants at high nitrogen and phosphorus concentrations were 2.78 percentage points and 18.56 percentage points higher than those at medium and low nitrogen and phosphorus concentrations, respectively, and the average removal rates of TP were increased by 4.24 percentage points and 10.63 percentage points, respectively. The entropy weight TOPSIS evaluation showed that the comprehensive evaluation was the highest when the combination mode of reed + cattail was planted at low density (15 plants/m2) under low, medium and high nitrogen and phosphorus concentrations, and the comprehensive benefits of nitrogen and phosphorus removal rate and economic cost were the best. The research results had a guiding role in the phytoremediation of reeds and cattails in northern farmland drainage ditches.