Outdoor air load plays a significant role in the air-condition system. Reducing the quantity of fresh air can be used to achieve the goal of energy saving as well as getting a comfortable environment indoor. The concentration of CO2 is used as the indicator of measuring the quality of air indoor and it becomes a new method of controlling fresh nowadays. The conference room is taken as an example, and is commutated with ACH method, Minimum air method and CO2 controlling fresh air method. The conclusions are as follows: If it is in the same situation,1. When there are less than 60 people, with the ACH method it costs the most fresh. That is, its energy consumption is the largest. 2. When there are 60~76 people, the CO2 controlling fresh air method needs the least fresh air. That is, it costs the minimum energy. The ACH method and the Minimum air method are all designed in accordance with the maximum load. So with these methods it can lead to a great waste of energy. 3. When there are more than 76 people, the former two methods can’t meet the health requirements. In the end the strategy of CO2 controlling fresh air method is given.