Abstract: The micromechanism of strength increase with curing time for compacted cement-soil was investigated by unconfined compressive strength experiment and microstructure observation. The results show that the unconfined compressive strength increases with increasing curing time, and the extent of the strength gradually decreases and then tends to steady. The strength of 60 d curing time can be used for engineering design of compacted cement soil. The change in strength with curing time substantively is cement hydration from weak to strong, ion exchange and agglomeration between hydrated gelatin and active substance of mixed soil, and induration reaction. The microstructure of hydrated crystals is from floccule and fibre-like to mum-like, finally to gridding. The pore among grains is from large to small. Cement hydration makes dispersed soil grains to agglomerate. With durative cement hydration, soil and hydrated substance agglomerations join each other to form cement-soil concretion by gridding hydrated crystals, which results in the increase of cement-soil strength.