In order to investigate the function and mechanism of the plants in constructed wetlands used for municipal sewage treatment, a pilot vertical\|flow wetland consisting of three sequenced cells (A, B and C) was established using Scirpus validus, Cyperus alternifolius, Canna indica, and Phragmites australis as the testing plant species. The results obtained from the study showed that the treatment efficiency in terms of average pollutant removal per unit area was consistently higher in A, B and C cells with plants than in relevant control cells without plants. In comparison, the increased amount of pollutant removal accounted for by the plant effect in A, B and C series was, respectively, 43.52 g/(m2?d), 20.38 g/(m2?d) and 30.94 g/(m2?d) for CODCr; 13.14 g/(m2?d)，28.61 g/(m2?d) and 6.97 g/(m2?d) for TN; and 1.2 g/(m2?d)，0.66 g/(m2?d) and 0.06 g/(m2?d) for TP. From cells A to C, the root activity increased while the height of plants, length of plant leaves, dry matter of plant growth increment, amount of N and P uptake and the content of peroxide enzyme all decreased significantly with the gradually decreased pollutant concentrations in the influent. The enzyme activity in the filler layers was low in the control and was significantly enhanced in the cells with plants, with a general increasing trend towards increasing root activity.