This study systematically investigates the effects of varying Cl⁻/${\mathrm{S}\mathrm{O}}_4^{2-}$ concentration ratios in saline solutions on the drying shrinkage, mechanical properties, and microstructure evolution of a cementitious system under simulated saline–alkali conditions. The underlying influence mechanism is elucidated via TG-DTG, XRD, and SEM analyses. Experimental results indicate that increasing the Cl⁻/${\mathrm{S}\mathrm{O}}_4^{2-}$ concentration ratio of the mixing water from 0.2 to 2.5 leads to a significant rise in early-age drying shrinkage, with an increase of approximately 19%, while it simultaneously enhances the early-age compressive strength of the cementitious matrix, achieving increases of approximately 13% at 1 d, 14% at 3 d, 14% at 7 d, and stabilizing at about 7% by 28 d. Microscopic characterizations reveal that as the Cl⁻/${\mathrm{S}\mathrm{O}}_4^{2-}$ concentration ratio increases, the ettringite content decreases, the contents of Friedel’s salt and calcium hydroxide increase, and the cementitious microstructure accordingly becomes denser. This work aims to provide theoretical and experimental references for the durability design and performance optimization of cement-based materials in saline–alkali regions.