A method for generating coincident design days by the heat balance method （HBM） was proposed. The hourly dynamic cooling load of a city over the years was calculated， the theoretical design load was determined based on no guarantee of 50 hours for an average year， and the coincident design day was selected by the comprehensive clustering method. The peak loads of the coincident design day and the traditional design day were calculated by the HBM and identified as the coincident design load and the traditional design load. The difference rates among the coincident design load， traditional design load， and theoretical design load of the sample rooms in Harbin， Beijing， Changsha， and Guangzhou were compared， respectively. The result shows that the deviation range of the difference rate between the coincident design load and the theoretical design load in each city is much smaller than the deviation range of the difference rate between the traditional design load and the theoretical design load. It indicates that it is more reasonable and accurate to calculate the design load of the air conditioning system based on the meteorological parameters of the coincident design day， and the coincident design day suitable for engineering application can be accurately generated by the HBM.