Abstract: The red layer residual soil is characterized by poor cementation, easy swelling, and softening upon contact with water. It is difficult to be directly used as an engineering filler. Theproposed solution is the use of cementitious capillary crystalline waterproofing materials(CCCW)as an external admixture for its improvement. Shear tests, scanning electron microscope tests, and X-ray diffraction tests were carried out on samples with different dry and wet cycles(0~7 times) to analyze the change in macroscopic mechanicalproperties, fine fissures, and microscopicpore characteristics of the improved soil. This was done to reveal the improvement mechanism of CCCW on the macroscopic mechanics of the soil body and its ability to resist wet and dry deterioration. The results show that:(1)The shear strength and cohesion of the improved soil increased and then decreased with the increase of CCCW dosage, and the optimal dosage of CCCW was 2%; (2)After 7 wet and dry cycles, the average fracture rate and fracture width of the improved soil with 2% CCCW decreased by 80.1% and 56.6%, respectively, compared with the CCCW=0(control group). This indicates that CCCW can inhibit the development of expansion and contraction fissures, maintain the structural integrity of the soil, and improve its resistance to wet and dry deterioration; (3)The complexation-precipitation reaction of CCCW promotes the generation of hydration products and the formation of a three-dimensional mesh structure consisting of CaCO₃ crystals, flocculent gel, and soilparticles lapped on each other, which plays the role of filler and skeleton in the soil body and improves the strength of the improved soil.