Owing to the high value of indoor heat gain in winter, the equipment rooms in an office building of Shanghai adopt air-conditioning system for cooling all the year around. For the actual demand of cooling load is low in winter, the equipments are often regulated by operating staff to increase the cooling load in order to ensure steady operation of the chillers. The scheme is determined by field research that utilizing free cold source, namely outdoor fresh air, to fulfil the cooling demands in intermediate season and winter of the equipment rooms which need cooling all year round. An improved IAQ will be obtained due to mechanical ventilation by using filtering devices of summer air duct system. Owing to reducing the year running time of related equipments such as chillers, chilled water pumps, cooling water pumps, cooling tower and so on, a large amount of air-conditioning energy consumption can be saved. On the basis of BIN parameters of Shanghai and the switch temperature defined in the paper, the energy consumption saved by free cold source and the operation consumption of air-conditioning are calculated respectively. The results show that the quantity of standard coal and electricity saved in a year are 703~795tce and 1.74×10⁶~1.97×10⁶kWh. Then the net total cost economized in first year is 1.39×10⁶~1.54×10⁶RMB. Not considering bank interest rate and price fluctuation, the investment return cycle is less than two months. Therefore, the reconstruction scheme is feasible and economical. In addition, this scheme can also be applied to the comfort air conditioned rooms especially to the room with the requirement of cooling all the year round.