By numerical stimulation the temperature distribution were obtained in block-stone embankment with block-stone in different thicknesses. The comparative analyses show that block-stone embankment has well active cooling effect on the railway foundation by raising the permafrost table and decreasing the permafrost temperature. For a certain construction environment and a certain construction design there is an optimal thickness of block stone which will freeze the foundation most strongly and raise the permafrost table maximally. The cause is the slope effect of embankment and the thickness of block-stone. When the block-stone is thin the air flow in embankment is weak and can not obtain great cooling ability;but when too thick the slope effect will cause multi-direction flow of air and weaken the cooling ability in winter and heat insulation effect in summer.