Abstract: Salt-induced deterioration of concrete is a critical factor affecting its long-term durability in saline soil environments. To enhance the service performance of concrete under such conditions, this study prepared basalt–polypropylene fiber reinforced lithium slag concrete (BPFLiC) by incorporating basalt fiber (BF) and polypropylene fiber (PF). The mechanical properties of BPFLiC after 180 d of immersion in simulated saline soil solution were systematically evaluated, and the evolution of pore structure during salt erosion was characterized by mercury intrusion porosimetry (MIP). Furthermore, the relationship between the fractal dimension of micropore volume and the macroscopic mechanical properties was analyzed. The results indicate that after 180 d of salt erosion, the compressive strength and splitting tensile strength of BPFLiC were increased by 17.71% and 39.35%, respectively, compared with those of plain concrete (RC); the capillary water absorption and porosity were reduced by 60.35% and 32.57%, respectively, and the most probable pore size was 65 nm. Overall, BPFLiC demonstrates significantly better resistance to saline soil erosion than RC.