Abstract: To address the problems of low strength, poor stability, and insufficient utilization of solid waste in the silty clay of the Yellow River floodplain, an alkali-activated slag-fly ash solid waste-based stabilizer was prepared. The mix proportions were optimized using response surface methodology, and its stabilization effect and mechanism were systematically investigated through macroscopic mechanical tests and microstructural analyses. The results indicate that the model showed high fitting accuracy, with the optimal mix ratio determined as 79.7% slag, a modulus of 1.58, and an alkali equivalent of 6%. This optimal ratio balanced both early- and later-age strength development. On this basis, the effects of stabilizer dosage and curing age on mechanical properties were further explored, demonstrating significant improvement. This enhancement was attributed to the dissolution of active components in slag and fly ash under alkaline conditions, followed by the gradual formation of C-(A)-S-H gels. These gels interconnect to form a spatial network structure, effectively filling pores and binding particles, thereby improving soil compactness. The alkali-activated solid waste-based stabilizer shows promising potential for enhancing the mechanical properties of silty clay, providing a theoretical foundation for its green stabilization and engineering application.