The time-varying distribution behavior of sulfate with diffusion is investigated for bored pile internal mixed corrosion at different amounts of service times. The formulation of diffusion, based on Fick-second law combined with the initial condition and the boundary condition, for sulfate in the bored pile is formed with the method of separation of variables. A model of concrete pile is offered as an example for illustrating the time-varying distribution behavior of sulfate and making comparison with the traditional model. Further, the influence factors and their effects on the sulfate diffusion distribution are also analyzed. The results show that the application of traditional method in analysis of sulfate diffusion in bored pile has some limitations and advantages of the proposed method are obvious. The radius of bored pile decreasing induces the discrepancy of sulfate distribution increasing with employing the traditional method, while the discrepancy increases over time. The time-varying distribution of sulfate in bored pile resembles hourglass figure that along the radius direction, diffusion behavior at x=0 and surface of pile is significant. In the region near x=0, the diffusion rate of sulfate is fast during the first year and then the diffusion rate is dramatically reduced to constant rate during the 50 years. Diffusion coefficient and the concentration gradient of sulfate in bored pile have a pronounced effect on the time-varying distribution of sulfate.