In rare-earth mining projects, ammonium sulfate (AS) solution has a great impact on the concrete structure, which often causes serious damage to the structure. To improve the corrosion resistance of concrete in AS solution, recycled plastic was used to replace concrete fine aggregate. Compared with normal concrete (NC), the deterioration mechanism of recycled plastic concrete (RPC) against the corrosion of AS solution (3, 5, and 7%) was studied. Through the tests and analysis of apparent morphology, relative mass, ultrasonic wave velocity, and compressive strength—as well as scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR)—of corroded concrete, the test results indicate that many spots and corner damage occurred in the early and later stages of corroded concrete, respectively. The corrosion reaction of AS solution produced more ettringite and gypsum, resulting in serious damage. The RPC was expansive under the corrosion of AS solution, and the expansion degree was greater than that of NC. The compressive strength of RPC decreased gradually in AS solution. The corrosion deterioration mechanism of RPC was revealed by microstructure and phase analysis.