Due to its poor engineering properties, silty soil with low liquid limit must undergo reinforcement treatment before it can be applied in practical engineering. Silica fume was used as the main cementitious material, and quicklime and sodium silicate were adopted as alkaline activators. The Box-Behnken method within the response surface methodology was employed to investigate systematically the solidification effect of solid waste-based cementitious materials on silty soil. Factor levels were determined through preliminary single-factor experiments, and the 7-day and 28-day unconfined compressive strength (UCS) were taken as key response indicators to analyze the effects of sodium silicate, quicklime, silica fume, and their interactions on the strength of the solidified soil. The results show that the content of sodium silicate and silica fume has a statistically significant impact on the UCS of the solidified soil, while the independent effect of quicklime on the UCS of the solidified soil is not particularly obvious. Specifically, the interaction term of quicklime-silica fume significantly affects the 7-day UCS of the solidified soil, and the interaction terms of sodium silicate-quicklime and sodium silicate-silica fume significantly affect the 28-day UCS of the solidified soil. The optimal ratio of the proposed curing agent is determined as sodium silicate∶quicklime∶silica fume = 10.44∶21.12∶30.21, and the predicted values of the 7-day and 28-day UCS are 1.15 MPa and 3.99 MPa, respectively, which are 51.3% and 252.3% higher than the strength of cement-stabilized soil.