Abstract: Objective Completely weathered carbonaceous shale soil, which is widely distributed in mountainous areas of southern China, is highly prone to disintegration upon wetting and exhibits very low strength. These characteristics make it a major factor triggering shallow landslides and soil erosion along highway slopes. To improve its structural stability while simultaneously achieving low-carbon and environmentally friendly performance enhancement, this study adopts a ternary composite modification approach using cement, fly ash, and polyvinyl alcohol (PVA). The objective is to optimize the mix proportion and systematically evaluate the resulting improvements in mechanical and hydraulic properties, thereby providing a green protection strategy for such problematic soil slopes. Methods The ternary composite modifier was prepared with cement, fly ash, and PVA. Orthogonal experiments were designed to optimize the mass ratio. Cylindrical specimens were fabricated and cured under standard conditions for 7 days. Both unmodified and modified soils were subjected to direct shear tests, constant-head hydraulic conductivity tests, immersion tests, and pressure plate tests to measure shear strength, hydraulic conductivity, water stability, and soil-water characteristics, respectively. Microstructural changes were observed using scanning electron microscopy (SEM). The recommended mix proportion was determined based on the 7-day unconfined compressive strength. Results 1) The orthogonal experiments revealed that the influence order of the three factors on the 7-day unconfined compressive strength was cement > PVA > fly ash, with cement exerting the most significant effect. The recommended mass ratio is 9% cement, 8% fly ash, and 0.9% PVA. 2) After modification, the cohesion increased from 27.1 kPa to 163.2 kPa, the saturated hydraulic conductivity decreased from 6.39×10-5 cm/s to 2.07×10-5 cm/s (a reduction of 67%), and the water stability coefficient reached 96%. The soil-water characteristic curve indicated that the water retention capacity was enhanced after modification, and the van Genuchten model fitting parameter α (air-entry parameter) decreased from 0.02095/cm to 0.01144/cm. 3) SEM observations revealed that the unmodified soil possessed a loose, porous fabric with very limited interparticle cementation. In the modified soil, abundant hydration products—predominantly calcium silicate hydrate (C-S-H) gel—were observed densely coating the particle surfaces. These products effectively filled the originally interconnected macropores and formed strong bonds between particles, resulting in a compact, aggregated microstructure dominated by micropores and closed pores. Conclusions The polyvinyl alcohol composite significantly enhances the shear strength and water stability of completely weathered carbonaceous shale soil, while reducing its hydraulic conductivity of completely weathered carbonaceous shale soil, effectively reducing rainfall infiltration and soil erosion. It is highly suitable for green slope protection and provides a theoretical basis for soil reinforcement and water conservation in similar engineering contexts. The modification successfully resolves the structural stability issues while achieving the targeted low-carbon and performance improvements.