Cushion layer replacement is currently an important method for preventing and treating salinized deterioration in the earthen sites' basal sapping zone. The water resistance, salt-dissipating property, and load-bearing capacity are key indicators for evaluating the effectiveness of capping replacement. In this study, gravel is an inorganic admixture for the calcareous soil base material. Prepared 19 soil samples through the cross-combination of gravel with three types of organic binders. These samples were subjected to tests on wave velocity, volume shrinkage, permeability, capillary rise, and consolidation. The suitability of the composite materials for reinforcing the rammed earth heritage capping layers was further studied using the AHP-CRITIC-TOPSIS method. The results show that: (1) The wave velocity and volume shrinkage rate of the samples increased over time and reached stability after 7 days. Moreover, the water resistance, salt dissipation, and load-bearing capacity of the samples were significantly enhanced; (2) Comprehensively evaluating the suitability of the composite materials for the capping layers in the earthen sites' basal sapping zone using six indicators: wave velocity, volume shrinkage rate, permeability coefficient, capillary water absorption rate, compression coefficient, and final settlement, it was found that the SMS-S: G7:3 mixture showed the best suitability. This study provides a reliable theoretical basis for the restoration of earthen sites in the arid areas of Northwest China.