Due to environmental erosion, concrete structures are easy to crack and damage, and appropriate additives can reduce damage and deterioration. In this paper, the influence of different content of magnesium oxide expansive agent on the corrosion resistance of concrete was studied, the mass loss and strength change of concrete which soaked in MgSO4 solution were tested. Combined with the microstructure changes, the sulfate corrosion resistance of the concrete mixed with magnesium oxide expansive agent was evaluated. The results show that the mass and mechanical properties of concrete specimens increase first and then decrease in the long-term sulfate immersion environment, and the sulfate damage rate can be reduced by adding magnesium oxide expansive agent and reducing the concentration of sulfate solution. Among them, on the one hand, the addition of magnesium oxide expansive agent refines the internal pore structure of concrete, reduces the rate of sulfate corrosion damage, and delays the damage cracking of specimens to form microcracks; on the other hand, magnesium oxide expansive agent fills the microcracks after cracking, blocking or reducing the sulfate continuous transmission channel, thus hindering the crack propagation.