This paper proposed an optimization control method for fresh air rate based on the objective function of energy consumption. By establishing the module of optimization control, a simulation analysis was carried out for an office building of Hong Kong on TRNSYS platform. The simulation result has indicated that, compared with minimum fresh air requirement control, the optimization control has not only saved the energy consumption of fresh air fan, but also reduced the energy consumption of refrigerators, chilled water pumps and cooling water pumps. The air-conditioning system with the optimization control can save 2.73×105 kWh in a year and the energy-saving rate reaches 17.1%. Its energy-saving effectiveness is remarkable. Especially in most time from November to March, the optimization control not only reduces the energy consumption of air-conditioning system, but also greatly improves indoor air quality.