Abstract: The pore structure and distribution characteristics have important influences on the physical and mechanical properties of expansive soils. The pore characteristics of the expansive soils before and after improvement treatment of lime-volcanic ash are analyzed using the NMR technology to obtain their change of pores and distribution laws. The change of internal cementation of the expansive soils before and after modification is analyzed by means of the scanning electron microscopy. The experimental results show that the number of pores in the range of 0~0.1 μm in diameter is significantly reduced, that in the range of 0.1~4 μm in diameter increases, and that in diameter more than 4 μm decreases as a whole. After modification of the expansive soils, a mixed structure with honeycomb structure, skeleton structure and spongy structure is formed, which improves the compactness of soil particles, reduces the connectivity of pores, and effectively inhibites the swelling and shrinkage characteristics of the expansive soils.