In order to study the hydrothermal characteristics of the sand soil replacement subgrade, the method of indoor freeze-thaw cycle test was used to investigate the hydrothermal changes at different depths of the sand samples with different gradations, and to compare and analyze the effects of coarse sand and fine sand on the silty clay in the lower layer. The results show that during the freezing process, the soil close to the cold end will have a “water increase phenomenon”, and the silty clay is more prone to water migration than the sandy soil. Both the replacement of fine sand and the replacement of coarse sand can inhibit the cooling of the lower silty clay and reduce the freezing depth, so as to avoid the frost heave and thawing of the subgrade. Compared with coarse sand, the replacement of fine sand has a better effect on inhibiting the downward transmission of negative temperature at the top, and the replacement of fine sand is more conducive to maintaining temperature stability. The freezing depth of the replacement of fine sand is smaller than that of the replacement of coarse sand, which is more conducive to inhibiting the frost heave of the subgrade soil and maintaining the internal water stability. The freezing characteristic curve of soil and the thawing characteristic curve of soil do not coincide, and the freeze-thaw characteristic curve of replacement fine sand is least affected by temperature at the interface of replacement. It is concluded that the replacement of sand can effectively avoid the occurrence of frost heave phenomenon of subgrade, and the effect of replacing fine sand is better than that of coarse sand.