The structure and function of activated sludge community were studied in an A/O pilot-scale plant treating actual domestic wastewater. FISH results show the feasibility and effectiveness of sludge population optimisation. A/O process short-cut nitrification with process control of DO and aeration based on DO and pH on-line sensors can be achieved, and gradually eliminated nitrite oxidising bacteria, and therefore achieved nitrogen removal via the nitrite pathway. It can be realized sludge population optimisation, improving nitrogen removal and saving operational cost at the best with on-line process control.