A2N is two-sludge system, by using the method that first bringing up the cultivation of denitrifying phosphorus removing bacteria (DPB) and nitrification biofilm separately then connecting them, which can start up A2N system successfully. Nitrification biofilm was cultivated in a sequencing batch reactor (SBR). After 30 days, NH4+-N effluent concentration steadily stayed below 0.5mg·L-1.In another SBR, the activated sludge for the enrichment of DPB is from the anaerobic tank, which was firstly operated under anaerobic/aerobic (A/O) condition. After 20 days, PAOs was successfully enriched. Then, the activated sludge was conducted under anaerobic/anoxic/aerobic (A/A/O) condition, maintaining the anaerobic time, gradually increased anoxic time and induced aerobic time. After 30 days DPB was successfully enriched, two phases totally take 50 days. The removal efficiency of total nitrogen and phosphorus are above 85 % and 95 %, so that A2N system was started up successfully.