The severe cold climate zone in China has a wide distribution range with complex climate conditions, which was divided into 3 subzones according to heating degree days based on 18℃ (HDD18). In order to investigate the rationality of the current second level index for dividing climate region for building thermal design, the annual cumulative heating loads of typical buildings in 61 cities in severe cold climate zone were simulated by EnergyPlus software. The climatic characteristics of several cities were compared and the relationship between HDD18 and annual cumulative building heating load were explored. The results show that the annual cumulative building heating load in low-altitude region is much higher than that in high-altitude region with similar HDD18. The annual cumulative building heating load is linearly correlated with HDD18 only in the regions with small differences in solar radiation intensity and summer temperature. Therefore, HDD18 is suitable for zoning in areas with small differences in solar radiation and summer temperature, which should be combined with the solar radiation to divide climate region in severe cold climate zone.