Based on the statistical theory, the paper collected routine monitoring data for 3 years of 11 municipal wastewater treatment plants with secondary treatment or enhanced secondary treatment in a northern city in China, and analysis of the discharge concentration distribution of COD showed a Delta-lognormal distribution. On this basis, referring to the formulation methods of water pollutant discharge limits used in the USA, the paper studied and assessed the discharge status of COD of the municipal wastewater treatment plants in the city, including the daily maximum and the monthly average discharge status, and came to the conclusion that the daily maximum discharge status met the first grade A class discharge standard (50 mg · L(-1)) required by the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002). Meanwhile, the assessment conclusion was obtained for the monthly average discharge status, and the conclusions were discussed. The assessment method and the conclusions in this paper might provide reference for formulation and revision of water discharge standards.