Abstract: Cement-soil is widely used in all kinds of geotechnical engineering projects for its easy construction and good performance. But in actual project, the cement-soil often suffered erosion from external environment, especially in coastal area, which changes its micro-structure and deteriorates its mechanical properties and durability. Cement-soil samples were produced and chemical etchant was prepared to simulate coastal environment. The SEM was used to obtain the micro-structure image, and the characteristics of granules and porous under different erosion conditions were analyzed quantitatively. Furthermore, the correlations between micro-structure and macro-strength were studied, by which the controlling mechanisms of micro-structure on mechanical performance was discussed. Finally, the damage variable represented by particle area ratio was introduced to represent the damage level of mechanical performance.