Argillaceous sandstone from a slope in Three Gorges Reservoir is investigated. A series of tests including triaxial compression test under four confining pressures, scanning electron microscope (SEM) is conducted on the sandstone samples subjected to cyclic wetting-drying in different pH conditions(pH=3, pH=7). The area ratio of skeleton and fractal dimension of images obtained by SEM under different pH conditions and different times of dry-wet cycle is obtained by MATLAB. The study shows that when samples suffer the same times of dry-wet cycle, the fractal dimension in acidic conditions (pH=3) is bigger than that in neutral conditions (pH=7). Also, it is indicated that the fractal dimension is proportion to water absorption rate and inversely proportion to skeleton area and cohesion. And the critical area ratio of skeleton of argillaceous sandstone subjected to alternation of wetting and drying is calculated to be 0.55. A concept of erosion degree is defined for argillaceous sandstone subjected to dry-wet cycles. Then a relation curve between the erosion degree of argillaceous sandstone and times of dry-wet cycle is obtained by fitting. Damage variable formula of cohesive force is derived. The study results provide theoretical basis for erosion of argillaceous sandstone in different pH conditions.