By carrying out water purification test on the sample diluent mixed by the tail water from Kunshan Sewage Treatment Plant, sewage purification effect of three submerged plants, namely Vallisneria natans, Vallisneria spinulosa and Potamogeton pectinatus, was studied in this research. For each plant, five gradients of plant density are set with a blank control group. After getting the test data, the impacts of different species and planting densities on plants’ sewage purification are analyzed from two aspects, namely pollutant-removal efficiency and instability index. The analysis results showed that: innovatively defined removal efficiency and instability index had good evaluation effect. These two indexes were respectively used to evaluate the trend of water quality change during the whole water purification test, and combined with each other to comprehensively evaluate the water ecological restoration effect of the full test cycle. The specific application has been well practiced in this paper. There were differences in the purification effects of different plants on the pollutants in the experimental sewage, and the planting density also had a certain degree of influence on the purification effects. Therefore, in the water ecological restoration project, it is suggested to select suitable submerged plant species and planting density, and pay attention to monitor the growth status of plants, so as to avoid water quality deterioration caused by vicious competition among plants.