The effect of C/N ratio on nitrogen and COD removal in combining surface flow and horizontal flow constructed wetlands under different influent nitrogen concentrations, as well as the preliminary influence mechanism was investigated in this simulation experiment. The results showed that, during the vegetative period and the flowering period, low C/N ratios were in favor of total nitrogen and ammonium-nitrogen removal in the wetlands, and nitrate-nitrogen removal was more complete with C/N ratio increasing, while nitrification in the wetlands limited nitrate-nitrogen removal under low C/N ratio. Comparing the two growth periods, the removal capacity of total nitrogen as well as nitrate-nitrogen was similar under different C/N ratios, whereas ammonium-nitrogen removal was higher during the vegetative period than during the flowering period. However, no obvious influence on COD removal by C/N ratio was found in all the treatments at the same growth period, and COD removal was also similar during the two growth periods, and their efficiencies in all the treatments were greater than 90%. With the increase of retention time, the removal efficiencies of both nitrogen and COD increased. The effluent pH value generally rose firstly and then kept a stable trend in all the treatments, while oxidation-reduction potential dropped firstly, rose secondly and then maintained a stable trend. The pH value decreased with C/N ratio increasing in the effluent, while oxidation-reduction potential increased. Besides, pH value was higher during the vegetative period than during the flowering period in the effluent, while oxidation-reduction potential was lower during the vegetative period than during the flowering period. Influent C/N ratio and the wetland system jointly affected the effluent pH and oxidation-reduction potential.