The effects of wet and dry cycling and sulfate erosion on the mechanical properties, physical phase changes and pore structure of cementitious materials were investigated by means of X-ray diffraction, Mercury intrusion porosimetry and thermodynamic simulation. The results show that after 120 d of coupled erosion, the formation rate of expansive erosion products, such as ettringite, is accelerated, which causes significant changes in the macroscopic and microscopic properties of cementitious materials. Specifically, the compressive and erosion resistance coefficients changed from increasing to decreasing, and the mass loss increased. The higher the water-cement ratio, the more erosion are produced. These products mainly accumulate in the transition pores of 10-100 nm, and the swelling effect causes the destruction of such pores, resulting in a decrease in their number. The average pore size increased from 45.31 nm at 120 d to 78.79 nm at 180 d, resulting in severe deterioration of the cementitious material after 180 d.