<p>Using waste steel slag as a geotechnical backfill material is an effective way to improve its utilization rate. However, unlike traditional soils, shallow-buried steel slag undergoes oxidation under humidity and air exposure. Furthermore, the pH of the backfill environment can significantly affect its mechanical properties such as in saline–alkali soil or acidic landfill leachate. Therefore, the effects of oxidation period and pH value on the dynamic properties of steel slag were investigated. The dynamic shear properties of the steel slag were determined using resonance column tests, while its microstructural characteristics were examined by scanning electron microscopy, X-ray diffraction, and nitrogen adsorption surface area analysis. The results indicate that the maximum dynamic shear modulus of steel slag increases with extended oxidation time, exhibiting a distinct triphasic pattern: slow growth at the initial and final stages, and rapid growth in the intermediate stage. Both acidic and alkaline environments were found to enhance the dynamic shear modulus of steel slag. This enhancement is attributed to the formation of hydration products including calcium silicate hydrate (C–S–H) gel, calcium hydroxide crystals, and ettringite (AFt) during the oxidation of steel slag upon exposure to air and moisture. The C–S–H gel and needle-like ettringite fill the interparticle pores, binding the particles into a denser and more integrated matrix, which ultimately improves the dynamic shear modulus. Steel slag can replace traditional fine sand in neutral environments, medium sand in alkaline environments, and coarse sand in acidic environments. For steel slag engineering backfill applications, alkaline sites are preferentially recommended.</p>