By monitoring the operation effects of a wastewater treatment plant in Handan,it was found that the removal rate of nitrogen was low and the effluent water was not up to the national standard for wastewater discharge.According to the result of the nitrogen removal,the research analyzed the distributional variation characteristics of dissolved oxygen(DO),mixed liquor suspended solids(MLSS) and the flow velocity at normal running conditions.The results show that the leading cause of the bad nitrogen removal is the deficiency of DO,which can not form the aerobic region;high concentration sludge directly reduces the concentration of dissolved oxygen and low flow velocity affects the delivering of dissolved oxygen.Therefore,controlling of sludge age effectively and strengthening the mixed impetus are the keys to improve the nitrogen removal.