Rock surface flow, formed from exposed rock surfaces, is a unique form of runoff in rocky desertified areas with massive bedrock outcrops. The question of whether the rock surface flow promotes surface runoff formation continues to puzzle us. To address this problem, four sites with notable bedrock outcrops and different land use types were selected from a typical desertified area in Guizhou, China. Straight and concave rock-surface shapes that were the most capable of collecting rainwater were selected at each site. Soil infiltration and water storage capacity experiments were conducted to observe the infiltration and water storage properties of the soil closely adhering to the rock–soil interface (CRSI) of the bedrock outcrops by taking the soils far from the outcrops horizontally (FRSI) as the control, and to further explain the conditions for surface runoff formation via rock surface flow. Our results showed that the CRSI is more prone to producing surface runoff than the FRSI, which is attributed to its lower infiltration (stable infiltration rate of 0.2 to 192.6 mm/min) and water storage capacity (total reservoir 441.6 to 613.6 t/hm2) than that of FRSI (4.50 to 272.8 mm/min, 458.8 to 635.4 t/hm2). This phenomenon would be intensified after receiving the rock surface flow input. Concurrently, concave rock surfaces are more likely to promote surface runoff formation than straight rock surfaces, which are affected by the accumulation of rock surface shapes on rainwater. A discriminating condition for whether rock surface flow produces surface runoff at the CRSI is proposed based on the inversely proportional functional relationship between rainfall (rainfall intensity RI and amount RA) and the effective rock surface rainfall catchment area (ARS). When RI or RA exceeds the critical threshold, the surface runoff with mechanisms of excess infiltration or excess saturation is generated around bedrock outcrops. The discriminating equation will help increase surface runoff prediction accuracy in rocky desertified areas with exposed bedrock outcrops. These results help understand the importance and role of rock surface flow hydrologic processes in karst areas.