Based on the fact that most karst basins are ungauged basins, a lumped karst hydrological model called the LKHM, which requires fewer data than typical models, was established in this study. According to the two-element and three-dimensional properties of karst basins, the LKHM was established based on the three-water sources of the Xin’anjiang model and improved in its calculation of confluence. The karst landform was classified into three types through a generalizability study: grooves, fractures, and pipes. The lag-and-route method was used to simulate the storage effects of grooves on surface runoff. Several linear reservoirs in series were used to simulate the storage effects of fractures with different diameters on subsurface runoff. All pipes were generalized into one large pipe in order to simulate the exchange of surface water and subsurface runoff. The LKHM was applied to the Pinghu Basin in Guizhou Province. The results show that the model has high accuracy, indicating that its parameters and structure are reasonable and the model is applicable.