Abstract: Demand response is an important measure to ensure the balance of power supply and demand, and the air conditioning system is an important resource. The energy consumption of fresh air load accounts for a high proportion of air conditioning systems, but the potential of fresh air load based on indoor carbon dioxide concentration to participate in demand response must be clarified. In this paper, taking the fresh air system of office buildings as an example, proposes short-term response and all-day response strategies aiming at the lowest hourly energy consumption and the lowest all-day electricity cost, and the corresponding response potential evaluation methods are established. The results show that indoor carbon dioxide concentration inertia is small, and the fresh air system has a fast adjustment speed which is an excellent short-term response resource. The energy consumption per hour can be reduced by 20%~100%, the lower the personnel density is, the greater the response potential is, and the average fresh air load can be reduced by 34% during the peak electricity price period in summer, taking the peak and valley electricity price as the response signal, the all-day operating cost can be reduced by 14%. On this basis, the influence of the upper limit of carbon dioxide concentration, personnel density, air change rate, and climate on the response potential are explored. Fresh air flexibility provides a new solution for buildings to participate in demand response, which has high application value for improving the contradiction between power supply and demand.