Assessment of treatment performance in the large-scale constructed wetland (CW) for secondary effluent purification remains limited. The aim of this case study was to therefore to investigate the long-term treatment capacity of organics and ammonium pollutants in a large-scale multi-stage surface-flow (SF) CW fed with secondary effluents from the wastewater treatment plants (WWTPs) in northern China. The results for two-and-half-year study period indicated that the water quality parameters including chemical oxygen demand (COD) and ammonium (NH4+-N) met the Chinese Grade III of Environmental Quality Standards. The mass reductions of COD and NH4+-N were 53% (4032 kg ha(-1) y(-1)) and 72% (511 kg ha(-1) y(-1)), respectively. However, there was a significant positive correlation between influent loads and treatment performance. The optimal loading of 2.5 gm(-2) d(-1) for COD and 0.3 gm(-2) d(-1) for NH4+-N could be recommended for designing the sustainable large-scale multi-stage SF CW wastewater treatments.