Abstract: The application of iron tailing powder to concrete as a mineral admixture promotes high-value use of solid waste and energy saving and emission reduction. However, most studies on the durability of iron tailing powder concrete only consider single conditions or accelerated conditions, and its deterioration characteristics under multiple environmental coupling remain unexplored. In this paper, an iron tailing powder admixture is applied to concrete. The relative dynamic elastic modulus and mass loss rate are used as performance indexes to study the deterioration process of iron tailing powder concrete under load–wet–cycling–sulfate coupling and to explore the influence of iron tailing powder content and load rate on the internal deterioration of iron tailing powder concrete. Scanning electron microscopy, X-ray diffraction analysis, and mercury injection revealed the damage and deterioration mechanism of iron tail powder concrete under coupling action. The results show that the proper addition of iron tailings benefits the synergistic hydration between iron tailings and slag powder to achieve the synergistic and complementary effect and improve the hydration degree of concrete. As a composite admixture, the mixture of the two considerably impacts the performance of concrete. The degree of damage and deterioration of the iron tailing concrete decreases and then increases with increasing iron tailing content. The sulfate corrosion resistance of concrete is optimal when the dosage is 50%. Adding a proper amount of iron tailing powder improves the compactness of iron tailing powder concrete, reduces the transport path of sulfate ions, improves the tensile strength of concrete, delays the appearance of microcracks caused by expansion products, and thus improves the corrosion resistance of iron tailing powder concrete. Under the coupled action, the expansion stress of ettringite and gypsum and the crystallization pressure caused by sulfate continuously increase the internal cracks of iron tailing powder concrete. Load application increases the width and number of microcracks in the test block. When the load rate exceeds 40%, the load accelerates the elastic deformation of cracks, reduces the effective bearing area of concrete, reduces the bearing capacity, and destabilizes the internal structure. At the same time, continuous calcium dissolution formed more channels inside the concrete and provided favorable conditions for infiltration and erosion by sulfate ions. The repeated dry–wet cycle caused the internal damage of the concrete to accumulate repeatedly, and the specimen was destroyed after reaching the ultimate strength of the concrete.