To address the deterioration of concrete exposed to high-sulfate environments, sulfate wet-dry cycling tests were conducted to systematically investigate the effects of different combined incorporation methods of polyester fiber, basalt fiber, and U-type expansive agent on the sulfate erosion resistance of concrete. By analyzing the deterioration patterns of specimen appearance damage, mass loss, and pore structure evolution, the sulfate erosion resistance of concrete was evaluated, and the synergistic mechanism by which fibers and expansive agent inhibit concrete deterioration was revealed.The results show that the concrete incorporating 0.15% basalt fiber by volume and 4% U-type expansive agent by mass of cementitious materials exhibited the best sulfate erosion resistance. After 50 cycles, the specimens remained intact in appearance, with no paste spalling observed. The mass loss rate was 6.12%, compared with 9.52% for the control group. The proportion of harmless pores decreased by 5.55 percentage points relative to that before the test, while that of the control group decreased by 10.00 percentage points. The synergistic effect of the expansive agent and fibers optimized the pore structure of concrete, thereby improving its impermeability and resistance to sulfate erosion.