To address the rapid deterioration of concrete durability under combined salt freezing erosion in cold-arid regions, this study designed experimental schemes with varying basalt fiber volume fraction and length through laboratory freeze-thaw tests. Using deionized water and a composite salt solution (3% (mass fraction) NaCl+5% (mass fraction) Na₂SO₄) as erosion media, this paper investigated the evolution of concrete’s durability properties during freeze-thaw cycles. Nuclear magnetic resonance (NMR) technology was employed to measure porosity variations in concrete specimens under different conditions, and a fractal dimension model was developed to analyze pore size distribution characteristics. Results show that basalt fiber incorporation effectively improves concrete’s mechanical properties and reduces salt-freeze erosion damage. Both relative dynamic elastic modulus and compressive strength initially increase then decrease with increasing basalt fiber content. Fiber incorporation reduces concrete’s internal porosity. The fractal dimension model indicates optimal pore structure densification occurs at 0.15% (volume fraction) fiber content and 25 mm length. Results from NMR tests and fractal modeling show good agreement with experimental data. The findings provide both theoretical basis and technical references for optimizing and maintaining hydraulic concrete in cold-arid regions.