Aiming at the erosion damage of backfill materials caused by sulfate solution erosion and dry-wet cycling, dry-wet cycling tests of backfill specimens were carried out in tap water and sodium sulfate solutions with mass concentrations of 3%, 6% and 9%, respectively. Meanwhile, the microstructures of the backfill were observed by scanning electron microscopy (SEM). The damage evolution law and erosion damage mechanism of the backfill were studied. The test results show that after 120 times of dry-wet cycling, the mass of the backfill decreases by only 0.19% in the tap water environment. However, the mass losses under the erosion of sodium sulfate solutions with mass concentrations of 3%, 6% and 9% are 0.73%, 1.52% and 3.64% respectively, and the mass reduction amplitude increases in a gradient manner with the increase of solution concentration. Under the condition of the same sodium sulfate solution concentration, the uniaxial compressive strength of the backfill specimens shows a gradual decreasing trend with the increase of the times of dry-wet cycling. After undergoing 30 times of dry-wet cycling of erosion in a 3% sodium sulfate solution, the uniaxial compressive strength of the backfill increases from 14.36 MPa to 14.85 MPa. The free sulfate ions in the sodium sulfate solution enter the backfill and chemically react to form ettringite. A small amount of ettringite is filled into the internal pores of the backfill, which enhances the mechanical properties of the backfill to a certain extent. However, with the increase of ettringite, the density of microscopic defects in the backfill increases, resulting in a gradual decline in its strength.