In order to explore the high-value utilization path of industrial solid waste in the field of road engineering, the bottom ash-slag-calcium carbide slag composite cementitious material was prepared based on the synergistic excitation theory, and the strength performance and stress–strain relationship of the three-slag cementitious material stabilized silty clay were analyzed. Based on the physical project, the test section was filled and the road effect was monitored and analyzed. The results show that there is a synergistic excitation effect between bottom ash and slag in the alkaline environment of carbide slag. The results of in-site monitoring data show that the stress conduction and the strain rate of the subgrade are stable. Therefore, it is proposed that the mass ratio of bottom ash: slag: calcium carbide slag: silty clay suitable for highway subgrade is 3: 8: 4: 85. It provides theory basis and engineering guidance for the preparation and application of cementitious materials by multi-source solid waste synergistic excitation.