Abstract: The preent artitle aims to study the denitrifying alaracter of simultaneous nitrification and denitrifieation (SND). Under the condition that C/N ratio was 12, this paper analyzed the regularities of triple-nitrogen and organic matter degradation in the process of simultaneous nitrification and denitrification during a single representative period, and studied the effect of technical parameters on the performance of denitrifieation in moving-bed biofilm reactor (MBBR). The results show that the phenomenon of simultaneous nitrification and denitrification take place in the moving-bed biofilm reactor (MBBR), denitrification effect is excellent, and the removal efficiency of TN is 90 %. The change law of ρ (DO), oxidation-reduction potential (ORP), pH and alkalinity during the reaction course were investigated, and the relationship between the parameters and contaminants was established. Meanwhile, the thesis studied comparatively the relation between simulation alkalinity and practical alkalinity during experiment. The results show that, in the moving-bed biofilm reactor (MBBR), alkalinity which, is produced by effect of simultaneous denitrification can supplement consumptive alkalinity of nitrification, and therefore alkalinity needn't to be supplemented in the process.