Chlorine dioxide (ClO2) is widely used as disinfectant in tertiary sewage treatment process. In order to identify the impact of disinfection with ClO2 on treated water quality, secondary effluent pretreated by coagulation was disinfected by ClO2, and the characteristic of dissolved organic matter (DOM) and the biological toxicity of the water before and after disinfection were investigated. The results showed that ClO2 could remove part of DOM and the removal rate of hydrophobic (HO) component by ClO2 was higher than that of hydrophilic (HI) fraction, resulting in the ratio of HI/HO increased. ClO2 could reduce the substitution degree of aromatic ring of DOM which could lead to the decrease of trihalomethane formation potential. After treated by ClO2, amount of DOM in every molecular weight (MW) interval dropped, implying that micro-organisms regeneration potential decrease. Furthermore, ClO2 could decrease the biological toxicity of urban wastewater. These findings may be useful in evaluating the feasibility of the ClO2 disinfection of tertiary sewage treatment.