Abstract: In order to realize the dual functions of water purification and flood discharge and drainage in the micro-polluted river, this study constructed a kind of bypass wetland with a downward-upward compound flow process. The main parameters of the project were organic load, hydraulic load and hydraulic retention time, which were 21 kg BOD₅/(hm²·d), 0.34 m³/(m²·d), 2.2 d, respectively. At the same time, the project was optimized by the structure of the wetland packing bed, the plant combination and the low-temperature operation measures in winter. The main conclusions were as follows:1) the downward/upward-flow composite functional wetland had a flexible spatial layout. The construction could save the land, and form a good Anaerobic/Anoxic/Aerobic microbial environment, which is conducive to the removal of nitrogen and phosphorus from the wetland system; 2) by adding the covering layer of thermal insulation filler, operating at low water level under low-temperature ice, and strengthening with low temperature-resistant plants/microorganisms, COD concentration, ammonia nitrogen concentration, and total phosphorus concentration of the effluent of the three bidding sections was 20.67 mg/L, 0.77 mg/L,0.17 mg/L, under their influent concentration of 30~40 mg/L, 1.5~2.0 mg/L and 0.3~0.4 mg/L in winter, basically meeting the water quality design goal under low-temperature conditions in winter.