Understanding the coupled influence and mechanisms of bedrock outcrops and underground fissures on soil erosion processes is crucial for managing karst desertification slopes. However, quantifying how runoff and soil loss respond to varying bedrock and fissure configurations has been challenging due to the intricate surface-subsurface hydrological interactions inherent to karst landscapes. To address their effects, we conducted controlled rainfall simulation experiments enabling precise manipulation of bedrock outcrop ratios (Rbo) and underground fissure rates (Ruf), and analyzed runoff, soil loss, hydraulic parameters, and hydrological connectivity characteristics across different combinations of Rbo and Ruf. Partial least squares path model (PLS-PM) elucidated the complex correlations among these variables and revealed the mechanisms by which Rbo and Ruf influence soil loss. Results revealed that soil loss increased linearly from 0.73 to 1.54 g m-2 s-1 with increasing Rbo at 3% Ruf, but exhibited fluctuating patterns at 5% Ruf, with the highest value (0.5 g m-2 s-1) occurring at 40% Rbo, indicating nonlinear bedrock-fissure interactions. Notably, no measurable sediment was detected in underground fissure runoff samples (detection limit: 0.01 g), highlighting the dominance of surface erosion pathways under high-intensity, short-duration events. Rbo induced critical hydrological shifts, including a 78-84% reduction in flow resistance, a transition from laminar to transitional flow regimes, and a 9-16% enhancement in hydrological activity, while collectively promoting rill clustering and localized scouring. PLS-PM identified hydrodynamic forces as the primary erosion driver (path coefficient = 1.03), with bedrock and fissures exerting strong indirect controls. These findings pinpoint 20-30% Rbo zones as critical erosion hotspots where synergistic hydrodynamic and connectivity effects peak, offering crucial guidance for targeted restoration strategies in karst hillslopes. This study provides a basis for improving karst erosion models (i.e., RUSLE, WEPP) and optimization of soil and water conservation strategies in fragile karst landscapes.