Abstract: A municipal wastewater treatment plant(MWWTP) in Beijing City based on the traditional A ²/O process was added with multi-stage A/O(MAO) to enhance the nitrogen and phosphorus removal effect. The effluent from different units of the MWWTP was studied, and the removal effects of the pollutants in the wastewater especially for dissolved organic matter(DOM) by the enhanced process were analyzed, by analyzing the basic water quality indicators, ultraviolet absorption spectrum and related parameters, and combined with multi-variate statistic analysis. After treatment of the MAO-enhanced process, the average removal rate of chemical oxygen demand, dissolved organic carbon, total nitrogen, total phosphorus, ammonia nitrogen were 95.14%, 89.70%, 94.53%, 97.26%% and 99.74%, respectively, which met first class A of Urban Sewage Treatment Plant Pollutant Discharge Standards(GB 18918-2002). With the addition of multi-stage A/O units, the removal rate of DOM was increased. At the same time, the molecular weight of DOM and the degree of agglomeration increased significantly; the stability of the compounds increased. The correlation analysis of water quality index and ultraviolet fluorescence spectrum shows that the removal of nitrogen and phosphorus is more obvious when the type of substituent is mainly aliphatic chain in the aromatic ring.