Rock-soil interface is one of the main paths to transform from the outcrop rock surface flow and surface runoff infiltrating into the underground fissure flow in the rocky desertification area. Among them, the rock-soil interface flow has posed a significant influence on the transformation of rainfall runoff, rapid underground leakage of water, and soil erosion or leakage on slopes. This study aims to explore the formation process and transformation mechanism of the rock-soil interface flow from the outcrops in the karst rocky desertification area. A series of artificial rainfall experiments were carried out after simulating a typical rock-soil interface from outcrops. Specifically, the driving factors were rainfall intensity, rock surface inclination, and rock surface shape. The test conditions were the surface slope, underground fissure density, and soil layer thickness. An investigation was made on the production and output characteristics of the surface runoff and such subsurface flows as the interflow, rock-soil and non-rock-soil interface flow under the rock-soil structure unit with the exposed rock surface and soil. Then, the formation of the rock-soil interface flow was analyzed to clarify the influence on the transformation of runoffs around outcrops. Results showed that: 1) The amount of flow at the rock-soil interface increased first and then stabilized during the rainfall process. The amount of rock-soil interface flow formed by rock surface flows from the outcrops (rock surface inclination surface were 45°, 60°, and 75°) was much greater than that by only the seepage flow in the soil (rock surface inclination was 90°) under the same conditions, where was 4.78 to 16.58 times of the latter. 2) There was little effect of rainfall intensity on the loss of the rock-soil interface flow. However, the rock surface inclination presented a significant impact on the loss of the rock-soil interface flow. The smaller rock surface inclination was, the greater the loss of the rock-soil interface flow was, in which the loss was more than 50% at the rock surface inclination of 45°. The rock-soil interface was directly formed by the rock-soil interface flow. A strong recharge was obtained on the non-rock-soil interface flow, where about half of the interfacial flow was eventually lost in the form of non-rock-soil interface flow. 3) The rock surface inclination was the main factor affecting the generation and distribution of rainfall runoff, followed by the rainfall intensity. The rock-soil interface flow contributed to the total water loss, and then decreased with the increase of the rock surface inclination, whereas, there was an increasing proportion of non-rock-soil interface flow to the total water loss. The surface runoff and interflow contributed little to the water loss. 4) Both the yields of rock-soil interface flow and non-rock-soil interface flow increased with the increase of rainfall intensity, but decreased with the increase of the rock surface inclination. The initial and stable flow generation times decreased with the increase in rainfall intensity. But there was no effect of the rock surface inclination on the initial and stable flow generation times. The time to form the stable production and flow path at the rock-soil interface was faster than that at the non-rock-soil interface. Once a stable path for water migration was formed, there was a rapid response to the changes in the rainfall intensity. The finding can also provide a theoretical basis for the generation and mechanism of runoff around outcrops in the rocky desertification area.