Abstract: An enhanced phosphorus removal biological aerated filter (BAF) treating petrochemical secondary effluent was used to investigate the optimized backwashing conditions. The optimized results can be used as the selection basis of backwashing BAF. The treated effluent of BAF was used as the backwashing influent, and the air-water combined backwashing mode was used. The selection of optimized backwashing conditions was based on the sludge concentration peak time of the backwashed effluent, specific oxygen uptake rate (SOUR) of the backwashed sludge, changes of the biomass and microbial activity, and the recovery of the BAF performance. The results showed that a water intensity of 6 L/(m²·s) and gas washing intensity of 12 L/(m²·s) were the optimized backwashing parameters. The best backwashing mode was:The best backwashing mode was:air backwash (air washing intensity 12 L/(m²·s)) for 4 min, air and water combined backwash (air washing intensity 12 L/(m²·s), water washing intensity 6 L/(m²·s)) for 5 min and rinse with water for 9 min. On these conditions, the sludge concentration peak of backwashed effluent had the shortest time of 3 min. The SOUR of the sludge in the backwashed effluent was relatively low, with an average value of 0.5 mg/(mg·h). The loss of sludge at the heights of 130 cm and 220 cm of the BAF were only 4.4% and 5.3%, respectively. However, the microbial activity measured by dehydrogenase activity increased slightly. Under the conditions, the particles trapped between the filter surface and the aging biofilm could be effectively removed. In addition, the active biofilm remained with the increasing mass transfer. The recovery of the BAF performance was very quick, and the ability to deal with pollutant removal could be recovered quickly after the backwashing. Thus, the optimized backwashing conditions of the reactor were determined.