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Free calcium oxide (f-CaO) in steel slag is converted to calcium hydroxide (Ca(OH)2) on contact with water, accompanied by significant volume expansion, a property that greatly limits the widespread use of steel slag. In this paper, a mixed solution of silica powder and sodium citrate was accustomed to treat steel slag to inhibit the volume expansion of steel slag, and the mineral composition and free calcium oxide content of steel slag were analysed by XRD. The findings indicated that the mixed solution of 3% silica powder and 0.2 mol/L sodium citrate had the best inhibition effect on the expansion of steel slag, and the mixed solution caused Ca(OH)2, the hydration product of f-CaO in steel slag, to be partly consumed by sodium citrate to produce CaC6H5O7, and partly produce calcium silicate (C-S-H) gel, which interacted with each other to form a dense layered structure on the surface of the slag to prevent further intrusion of moisture into the aggregate, and further increase the volume of the aggregate, and prevent the expansion of the slag. The interaction between the two forms a dense layer structure on the surface of steel slag, which prevents further moisture intrusion into the aggregate and further improves the comprehensive performance of modified steel slag.