Abstract: Expansive soil consists of montmorillonite and illite,which are all clay minerals with strong hydrophilicity,so it is a kind of clay with high plasticity,and has characteristics of high swelling or shrinking deformation,crack and over consolidation In the construction of the railway, expansive soil can not be used as the road bed directly, without being improved. The change regularity of the mechanical character of lime improved expansive soil under the influence of the various factors is discussed