Abstract: Based on the water, steam flow equation and heat diffusion equation, a subgrade model was established and the moistureity change rule of subgrade under the action of the atmosphere was studied. By changing  the atmospheric  phase parameters and the soil characteristics parameters, the unsaturated roadbed moistureity change rule under the influence of different climate environment was studied. The results show that the effect of atmosphere on the unsaturated soil subgrade is significant, and the depth of the  impact range is limited, the subgrade moistureity change caused by solar radiation is  the largest, the relative moistureity is the second largest; and the influence of the permeability coefficient and soil water characteristics of the subgrade itself on the filled soil moistureity change is very large; the higher the initial water content of the filled soil subgrade, and the bigger the evaporation strength, the higher the subgrade dehydrate degree; the rainwater will easier invade the subgrade, and the moistureity of the subgrade will be changed at long time and small rain rainfall pattern.

Key words: heat diffusion equation, hot and moisture coupling, solar radiation, moisture, rainfall pattern