To investigate the pollution status and migration and transformation rules of dissolved organic nitrogen (DON) in urban wastewater treatment plants (WWTPs) of Taihu Basin, several typical WWTPs were selected. Through long-term wastewater quality monitoring of the DON pollution level and seasonal variation, DON molecular weight, hydrophobicity, and fluorescence spectrum characteristics were studied by multiple methods. Experimental results showed that 97.66%-99.22% of effluent nitrogen were dissolved nitrogen compound, and the proportion of DON to soluble total nitrogen (STN) was 21.88%-26.15%, and the average DON concentration ranged from 2.7 to 3.4 mg/L. A significant decrease of DON was observed in anaerobic tank, and the DON with molecular weight higher than 30 ku was degraded to lower than 3 ku after moving bed biofilm reactor (MBBR) process. In the study, 42% of the DON with molecular weight lower than 1 ku was measured within effluent, and 79% of the effluent DON were hydrophilic compounds. Fluorescence spectrum showed that tyrosine-like proteins and soluble microbial metabolites were the main influent DON components, and the total fluorescence response percentage (P_i,n, %) reached 76.5%. However, the removal rates of tyrosine protein and soluble microbial metabolite were 77.0% and 23.7% respectively, which further indicated that the activated sludge process was not suitable to the effective removal of low molecular weight organic nitrogen.