This paper constructs a reactive-transport model for concrete under sulfate attack based on Fick's diffusion law and the principle of system Gibbs free energy minimization. Through a MATLAB interface program, COMSOL and GEMS are coupled to achieve the sequential non-iterative numerical solution of this model. The study examines the phase composition, porosity, and spatiotemporal distribution of ions and elements in concrete after sulfate attack considering calcium leaching. The results indicate that concrete expansion cracking, causing a loss of S-containing solid phases, is the reason for the peak distribution of sulfur elements. Due to the poor high-temperature stability of expansive corrosion products, the porosity of concrete near the attack surface decreases as the temperature increases. Furthermore, unlike the mass loss caused by expansion cracking at 23°C, mass loss of concrete under high-temperature conditions is primarily due to the decomposition of hydration products caused by a decrease in initial pH.