Expansive soil, as an unsaturated cohesive soil, is a harmful geological soil due to its nature of bulging in wetness and shrinking in dryness. Especially in the environment of alternating wet and dry climate, it would cause engineering accidents because of its nature. To solve this problem, expansive soils were modified with steel slag powder and cement to improve the performance, and the mechanical variation was analyzed under dry-wet cycles. In laboratory, four differently formulated expansive soils were tested, i.e., pure expansive soil (Es), cement modified (Es-C), steel slag powder-cement modified (Es-SSP-C), and steel slag powder-cement-NaOH modified (Es-SSP-C-N). Their variations in unconfined compressive strength were determined under different curing ages and dry-wet cycles. Results show that the strength of the three modified soils increased with the increase of curing age, and the four soils have different degrees of strength loss under dry-wet cycles, but the strength was always in an order of Es-SSP-C-N > Es-C > Es-SSP-C > Es, which means that Es-SSP-C-N was the best among them all.