In order to analyze the effect of external sulfate attack on the durability of concrete cast-in-situ piles, the diffusion-reaction behavior of sulfate ion for concrete cast-in-situ piles was studied and the factors that affect the degradation of pile structure were analyzed. Based on the Fick’s second law, the non-steady diffusion-reaction equation of sulfate ion under column coordinates was established. The finite difference scheme for solving the diffusion equation was obtained by the numerical method. The accuracy of the theoretical model and the calculation method was verified by related experimental results. Expansion strain was obtained according to the expansion product produced by chemical reaction. The damage and bearing capacity of pile were calculated by damage evolution equation. The correlation between damage degree and bearing capacity was established. Influences of different corrosion factors and corrosion time on the damage and bearing capacity of pile were compared and analyzed. Results indicate that the internal expansion strain and damage developed more rapidly on the surface than in the interior layers. The increase of pile radius could significantly improve the durability and bearing capacity of the pile. When the water-cement ratio was larger than 0.4, the damage degree of the pile experienced the biggest growth. Therefore, the reasonable increase of pile radius and the use of smaller water-cement ratio can effectively reduce concrete damage, improve pile durability, and reduce the loss of bearing capacity of pile.