Abstract: Abstract: This study explored the influence of design parameters on hydraulic performance and pollutants purification effect of free water surface flow constructed wetlands and selected the optimal design comprehensively considering the hydraulic and pollutants purification effects. An orthogonal test with multi-factor and multi-level of design parameters were performed to access their effects. The test site was located in the experimental base of the Jiangxi Province Center Station of Irrigation Experiment, in the Ganfu Plain of Poyang Lake basin. The geographical coordinates of the basin are 115°49′-116°46′E, 28°24′-29°46′N, characterized by a typical subtropical and humid monsoonal climate with an annual average temperature of 18.1 °C and an annual average rainfall of 1 685 mm. The orthogonal design, L16 (44×23), consisted of 4 4-level factors, namely aspect ratio, water depth, plant spacing and flow rate, and 3 2-level factors that are in- and outlet layout, plant species and blank. The hydraulic performance was analyzed by analyzing tracing data with rhodamine WT as the tracer. The hydraulic index used here were the short circuit indicator, the mixing indicator, the effective volume ratio, and the moment index. The pollutants purification effect of each test plot was obtained through multiple sampling tests of equal time intervals during the test. The detected pollutants were total nitrogen, total phosphorus, and total suspended solids. This test achieved a good purification effect on pollutants, the average concentration removal rate of total nitrogen, total phosphorus and total suspended solids was 40.0%, 58.3%, and 40.3%, respectively. The results showed that there was a significant correlation between aspect ratio and most hydraulic and treatment performance indicators. Water depth had a significant effect on total nitrogen removal rate, and plant spacing had a significant impact on total suspended solids removal rate (P<0.1). The correlation analysis between evaluation index showed that there was a universally strong significant correlation among the hydraulic performance indicators and also a significant correlation among the pollutants purification effect index. The better the hydraulic performance would have the higher pollutants purification effect. According to range results, aspect ratio, water depth, and plant spacing were the main design parameters that affected the hydraulic and pollutants purification effect of free water surface flow constructed wetlands. The results of variance showed that the design parameters had a significant effect on the pollutants purification effect, but the influence on the hydraulic performance was not as significant. For the evaluation of hydraulic performance, the optimal combination of design parameters consisted of an aspect ratio of 8:1, a water depth of 30 cm, a plant spacing of 30 cm, a flow rate of 0.55 m3/h, a layout of central pass in- and outlet, and Typha latifolia as the plant species. For the evaluation of pollutants purification effect, the optimal combination of design parameters consisted of an aspect ratio of 4:1, a depth of 30 cm, a plant spacing of 20 cm, a flow rate of 0.55 m3/h, a layout of central pass in- and outlet, and Scripus tabernaemontani as the plant species. Combining the results of the hydraulic and pollutants purification effect, the design parameter combination composed of an aspect ratio of 4:1, a depth of 30 cm, a plant spacing of 20 cm, a flow rate of 0.55 m3/h, a layout of central pass in- and outlet, and Scripus tabernaemontani as the plant species, resulted in an optimal overall performance. Since an excellent purification capacity was the ultimate goal for wetland design, the optimal design parameter combination for overall performance was the same as that for pollutants purification effect in this study.