The aim of this study is to investigate the durability of concrete with fly ash as a fine aggregate (CFA) in chloride salt environments. The natural diffusion method is used to analyze the concrete strength, chloride ion concentration, and diffusion coefficient after chloride salt erosion in a laboratory environment. In addition, scanning electron microscopy, X-ray diffraction, and Mercury intrusion porosimetry are performed to clarify the deterioration mechanism of concrete in chloride salt environments. The results indicate that owing to the chloride ion binding effect, the concrete porosity decreases and the concrete strength increases after chloride salt immersion. The decreased porosity caused by delayed hydration is primarily due to Friedel’s salt and CaCO3 produced during chloride ion erosion. CFA has a lower chloride diffusion coefficient and lower chloride concentration compared with ordinary concrete at all depths. In general, CFA exhibits excellent resistance to chloride salt erosion.