In this research, NO3--N was added into the typical sludge anaerobic fermentation (TAF) of waste activated sludge (WAS), creating an anoxic environment for the growth of denitrifiers, as well as the inhibition of methanogens, therefore to insure the operation of simultaneous anoxic fermentation and denitrification (SAFD) system. Compared to TAF, SAFD can utilize the endogenous carbon source of WAS to denitrification, which achieved higher sludge reduction. Results showed that the SS, VSS in SAFD was reduced by 63.8% and 74.7%, respectively, which is obviously higher than that of 36.7% and 47.6% in TAF. Volatile fatty acids (VFAs) in TAF reached the top with propionic acid as dominant component in the mid-period of the fermentation; whereas that in SAFD was preferred by acetic acid in the late period. Evidenced by the variation of TN, NH4+-N and NO3--N, SAFD own a significant ability in denitrification, while the protein conversion appeared to be continuously low in this system.