Abstract: The article discusses the characteristics of nitrogen removal in sequencing batch reactor of submerged biofilm process. It also elaborates the nitrogen removal mechanism of over-storage-SND. Nitrogen removal in the anaerobic phase depends largely on the over-storage function of the carbonaceous and nitrogenous organism by biofilm, and nitrogen removal in the aerobic phase depends on the SND function of biofilm. The organic carbon over-storaged in the anaerobic phase in the biofilm provides the major organic carbon in the process of denitrification.