Based on the mechanism of sulfate attack, a microstructure evolution model of hardened cement paste under sulfate attack was established by using the improved CEMHYD3D hydration model and random probability method. At the microscopic level, the free diffusion, random collision and transformation reaction of sulfate ions in the pore solution of hardened cement paste were simulated, and the microstructure damage and volume expansion induced by the growth of expansive erosion products were analyzed. Meanwhile, the content of gypsum and ettringite and the expansion strain of paste in the process of sulfate attack were calculated, and the model was verified by comparing with the experimental results in present literature. On the basis of the model, the microstructure evolution and expansion process of hardened cement pastes with different water-cement ratios under sulfate attack were numerically simulated. The results show that under the same sulfate concentration, the smaller the decrease in the contact area of calcium hydroxide and phase containing aluminum with pores is, the lower the expansion strain of hardened cement paste is. For hardened cement pastes with water-cement ratios of 0.25, 0.30 and 0.35, their expansion strain begins to increase rapidly when the pore filling degree reaches 9.09%, 9.27% and 9.41%, respectively. As the sulfate concentration increases, the time for rapid expansion of hardened cement paste is advanced.