Abstract: To investigate the deterioration mechanism of the microstructure of cement mortar under force-acid coupling erosion, this study employed low-field nuclear magnetic resonance (NMR) technology for non-destructive testing. The testing was conducted on acidic solutions with pH values of 3 and 5, as well as on samples subjected to 10% uniaxial compressive strength coupled with acid erosion. The deterioration mechanism was analyzed using pore size distribution, fractal theory, and pore sensitivity. The results indicate that acid erosion substantially compromises the stability of the pore structure. Under loading conditions, large pores are more prone to compaction, leading to the formation of microcracks. This process increases the proportion of gel pores while decreasing the average pore radius. Gel pores and capillary pores exhibit distinct sensitivities to the acid environment. Capillary pores within the range of 0.4–2.0 ms demonstrate positive sensitivity, whereas those in the range of 2.0–10.0 ms exhibit negative sensitivity. Force-acid coupling enhances the sensitivity of capillary pores but diminishes that of gel pores. Fractal dimension analysis reveals that both acid erosion and force-acid coupling reduce the fractal dimension of the pore structure. Notably, the coupling effect induces the most significant simplification of the pore structure, suggesting that loading exacerbates the complexity of the acid-eroded pore system. These findings provide a theoretical foundation for assessing the durability of cement-based materials in complex environments.