The cement and industrial solid wastes (fly ash and steel slag) are utilized to collaboratively solidify and improve the expansive soil. The effects of curing agent proportion, dosage, compaction degree and age on the mechanical properties of the solidified soil are systematically analyzed, and its potential application in subgrade engineering is explored. The results indicate that the active components of fly ash and steel slag in the curing agent raw materials can synergize the cement hydration reaction. The A₄ mix proportion (fly ash to steel slag ratio of 30∶45) of the solidified soil specimen is the best curing effect. At this point, the 28-day unconfined compressive strength of the solidified soil can reach 3.74 MPa. Increasing the curing agent dosage and upgrading the compaction degree can significantly improve the dry density of the solidified soil and reduce its optimal moisture content. With the increase of curing age, the hydration reaction of the curing agent can provide more hydration products to fill the voids of solidified soil and to improve its strength. The study confirms that cement-solid waste-based curing agent can effectively enhance the mechanical properties of expansive soil, providing the theoretical support for green reinforcement of subgrade and the resource utilization of industrial solid waste.