Abstract: The research of seasonal removal regularity of organic matter(chemical oxygen demand, COD) in lakeside constructed wetland can provide references for the design and operation of similar wetlands. Wangjiazhuang constructed wetland(WCW), with surface area of 12000 m², located at the eastern lakeside area of Dianchi Lake, Yunnan Province. WCW with dominated helophytes(Zizania caduciflora and Phragmites australis) has been put into operation since August 2002. The inflow from five agricultural ditches converged at a water distribution ditch, and then equally flew into WCW. WCW showed better capability for COD removal from agricultural runoff between October 2002 and June 2004. Thus WCW effectively cut the external COD load of Dianchi Lake. The average inflows of wetland in dry season (from November to next April) and rainy season (from May to October) were 497.6 and 747.4 m³/d, respectively. Influent COD_Cr loads during dry season and rainy season were 36.1 g/hm²/d and 67.9 g/hm²/d, respectively. The average influent and effluent COD_Cr concentrations and water temperature were 87.1 mg/L, less than 45.1 mg/L and 17.3℃ during dry season, and 109 mg/L, 53.7 mg/L and 21.9 ℃ during rainy season. The effluent water quality can meet the standard A and B of grade I discharge standard of pollutants for municipal wastewater treatment plant during dry season and rainy season. The average COD_Cr removal rates were larger than 48.2% during dry season and 51.0% during rainy season. The COD_Cr concentration decreased along the flow path of WCW. The COD_Cr concentration difference between the foreside and rearward of WCW was lower in dry season than that in rainy season, because water of Dianchi Lake had higher influence on the COD_Cr removal capability of WCW in dry season than that in rainy season. The pH value of top soil(with depth of 0～40 cm) in WCW increased along the flow path, but organic matter content decreased along the flow path.