Soil water and nutrient loss are not only the main reasons for degradation of land quality and declines in productivity, but also important factors contributing to the pollution of underground water in karst regions. Compared with non-karst areas, the special structure of karst slopes requires a different theory regarding soil nutrient loss. Previous studies have determined that rainfall is the primary external factor resulting in soil water and nutrient loss on slopes. Hence, determination of the mechanism of soil erosion and nutrient loss will allow for the elucidation of output characteristics of soil nutrients from karst slope farmlands under different rainfall intensities. In addition, understanding the direction and vehicle of nutrient loss could clarify the method of soil nutrient transfer on karst slopes. However, it is difficult to directly observe underground runoff and sediment on karst slopes (i.e., there is no feasible research method). Using karst slope farmlands as an example, the surface micro-topography and structural characteristics of underground pores and fissures were simulated in a laboratory based on field investigations on slope farmlands in the Guizhou Province karst areas. The goal of this study was to determine the characteristics of nitrogen, phosphorus, and potassium loss in runoff, as well as the sediment yield from surface and underground in karst slope farmlands under different rainfall intensities using simulated rainfall experiments. The results indicated that:1) For light (50mm/h) and moderate rainfall intensity (70mm/h), runoff on karst slopes was primarily dominated by underground runoff; for heavy rainfall intensity (90mm/h), both surface and underground runoff were nearly equal, with the surface water runoff being slightly higher than that of underground runoff. Sediment yield changed from equivalent surface and underground sediment under the light rainfall intensity to surface sediment production being dominant at greater rainfall intensities. 2) The concentration of each nutrient in runoff exhibited an initial scouring effect during the process of rainfall erosion on karst slope farmlands. Influenced by the effect of soil absorption, the effect of rainfall intensity on total potassium (TK) and total nitrogen (TN) concentration in runoff was substantially greater than that of total phosphorus (TP). 3) The output load for each nutrient in surface runoff, surface sediment, and total sediment increased with increasing rainfall intensity. The output load of TK was dominated by sediment, but those of TN and TP were dominated by runoff on karst slope farmlands. With respect to runoff output, the output load of both TN and TP were dominated by underground runoff. The percentage of the TP load in the surface runoff relative to total runoff ranged from 11.6% to 46.2% and that of TN ranged from 7.0% to 48.5%; however, the output load of TK was equal between the surface and underground water, with the percentage of surface runoff relative to total runoff ranging from 43.5% to 57.0%. The sediment load for all nutrients was dominated by surface sediment, and the percentage of each nutrient in the surface sediment relative to total sediment was more than 54.5%. These results could provide basic parameters and a scientific basis to initiate control for soil water and nutrients loss on karst slope farmlands.