This study investigates the damage and fracture characteristics of borehole surrounding rock under the influence of working fluid immersion. Numerical simulations using the Particle Flow Code (PFC2D) were conducted to analyze the mechanical properties of coal samples after exposure to supercritical CO₂ and active water. The discrete element model was calibrated based on experimental data to accurately simulate crack initiation, propagation, and coalescence under various fluid pressures and in-situ stress conditions. Results revealed that fracture propagation is highly sensitive to both fluid pressure and stress anisotropy. An optimal fluid pressure range was identified to minimize rock damage, beyond which fracture intensity increases. The study offers practical guidance for optimizing drilling fluid parameters to mitigate formation damage.