Orthogonal tests with mixed levels of design parameters of a free water surface flow constructed wetland were performed to assess their effect on hydraulic and treatment performance, and discover the relationship between the design parameters and the two performances. The results showed that water depth, plant spacing, and layout of in- and outlet mainly affected the two performances. Under 40 cm depth, central pass of in- and outlet, 1.8 m(3)/h flow rate, 20 cm plant spacing, 2: 1 aspect ratio, and Scripus tabernaemontani as the plant species, treatment performance of 5.3% TN, 6.1% TP and 15.6% TSS removal efficiencies and a high hydraulic performance of 0.854 e, 0.602 MI were achieved. There was no significant correlation between the design parameters and the two performances. The relationship among various hydraulic indicators and that among the purification indicators displayed extremely significant correlation. However, there was no significant correlation between hydraulic and treatment performance. (C) 2017 Elsevier Ltd. All rights reserved.