Abstract: This study investigates the coupled effects of slurry seepage and soil softening on borehole stability during horizontal directional drilling (HDD) reaming, through integrated experimental and theoretical approaches. Direct shear tests were conducted to quantify the softening of shear strength parameters (cohesion c and internal friction angle φ) under varying slurry viscosity and intrusion content, and a predictive model linking strength parameters to slurry properties was established. The strength softening law is incorporated into cavity expansion theory, and combined with the effect of seepage body force, a governing equation for borehole wall stability under seepage-softening coupling is developed. Analytical solutions are derived for cavity expansion ratio, plastic zone radius, and ultimate grouting pressure. Parameter analysis shows that slurry infiltration content is the dominant factor controlling soil softening, and once it exceeds 15%, borehole wall stability deteriorates sharply. Increasing slurry viscosity can suppress cavity expansion and plastic zone development to a certain extent, whereas high formation permeability intensifies the seepage-induced softening effect and further degrades borehole wall stability. The findings provide a theoretical basis for optimizing grouting parameters and construction control in HDD engineering.