Based on the investigation and statistics of karst slope near-surface karst conduits, this paper established a test micro-area of surface-underground binary space water and soil process in Karst area. Through artificial simulated rainfall test, the influence of pore size (1 cm, 2 cm, 5 cm) of karst conduits on water and soil leakage process under different rainfall intensities (52 mm/h and 133 mm/h) was studied. The results showed that:(1) The coefficient of underground runoff and the intensity of runoff increased significantly with the increases of pore size of karst conduits, but the effect on the initial runoff time was not significant. Under the moderate rain intensity (52 mm/h), the time required for initial runoff generation increased with the increases of pore size. However, under heavy rain (133 mm/h), the initial runoff time decreased with the increases of pore size. (2) With the increases of pore size, the characteristic curve of runoff intensity under moderate rain intensity (52 mm/h) showed that it increased first and then tended to be stable. The runoff intensity at mid-slope position was proportional to the pore size, and the upstream and downhill positions were not significantly affected by the pore size. The change order of runoff intensity with the pore size was 5 cm>1 cm>2 cm. The runoff intensity under heavy rain (133 mm/h) showed a continuous upward trend with the increases of pore size, and the gross runoff intensity at 2 cm and 5 cm aperture increased by 75% and 74% respectively compared with that at 1 cm. (3) Underground sediment yield of different karst conduits was proportional to their pore size. Under medium rain intensity (52 mm/h), the sediment yield of 2 cm and 5 cm karst conduits with pore diameter was 2.5 and 14.9 times of that of 1 cm respectively. Under heavy rain (133 mm/h), the sediment yield of 2 cm and 5 cm karst conduit with pore diameter was 6.9 and 10.3 times of that of 1 cm respectively. (4) Under heavy rain (133 mm/h), the order of mass loss of sediment in different particle size ranges was:particle size less than 2 mm>3~2 mm>5~3 mm>5 mm or more. These research results are helpful to further understand the influence of karst pore size on underground leakage in southwest karst area and provide reference for water and soil erosion prevention in this area.