In order to study the effect of Elodea nuttallii-Bellamya aeruginosa combination on lakeside wetland purification and to explore the best combination of density, by combining different density of Elodea nuttallii (100 g/m2, 200 g/m2, 300 g/m2) and Bellamya aeruginosa (120 g/m2, 240 g/m2, 360g/m2), the growth of Elodea nuttallii, the survival rate and growth of Bellamya aeruginosa under the condition of different combination proportions are studied . Mass concentration changes of TN, TP, COD, NH4+-N, NO3--N and NO2--N in water, and changes of TN, TP and COD in bottom mud are tested. The result shows that Elodea nuttallii-Bellamya aeruginosa combination has a great effect on removal of N, P and COD in water and bottom mud. The purification efficiency is the highest when the density of Elodea nuttallii is 200 g/m2. In this study, there is no significant difference in the final biomass (554.31±29.60g) of Elodea nuttallii. Higher density of Bellamya aeruginosa has a better effect on removal of COD, while the density of 360g/m2 causes higher mortality. The study shows that under the condition of this experiment, the best combination of density is Elodea nuttallii: 200 g/m2, Bellamya aeruginosa: 240 g/m2.