In large cities， the air conditioning load is an important component of power load. The air conditioning load consumes large amounts of electrical energy. The reduction in the air conditioning load is not only the crucial direction of electricity-saving， but also an important direction of demand side management. The air conditioning load is sensitive to air temperature， and there obviously exists the accumulative effect. The accumulative effects of high temperature and low temperature seasons are studied. A mathematical model for quantitative post-assessment of the accumulative effect is proposed. By an analysis of the relationship between air temperature and power load on the days without accumulative effect and a comparison with the actual situation， the electric energy consumption due to the accumulative effect can be obtained. The case study data on a city in North China shows that the energy consumption of air conditioning due to the accumulative effect is great， which is a tremendous potential direction of energy-saving.