Influent chemical oxygen demand (COD)/nitrogen (N) ratio plays a crucial role in the biogeochemical cycle of carbon and N in constructed wetlands (CWs). This study evaluated the greenhouse gas (GHG) emissions, pollutant removal efficiency and specific microbial gene abundances in subsurface flow CWs (SSFCWs) under various influent COD/N ratios. The GHG emissions and pollutant removal efficiency were markedly affected by influent COD/N ratios. The effluent concentrations of nitrate decreased with the increase of influent COD/N ratios, while the trend was completely reversed for COD. The lowest effluent total nitrogen concentration was observed with a COD/N ratio of 10. For GHG emissions, the CO2?fluxes increased with increasing influent COD/N ratios. However, the lowest CH4?(- 42.49?μg/m2/h) and N2O fluxes (31.77?μg/m2/h) were both observed under a COD/N ratio of 10. A significantly positive correlation between N2O fluxes and abundance of?nirS and (nirS?+?nirK) (r2?=?0.64 and 0.61,?p?