•A new distributed Karst Tunnel Hydrological Model (KTHM) was successfully developed.•Runoff simulated effects by the KTHM were satisfactory in karst areas.•Tunnels have the greatest influence on dry season runoff and the least influence on the flood. Tunnel excavation in karst areas could affect natural karst hydrological processes and runoff, resulting in groundwater drainage and even underground river cut-off. The hydrological model is a good tool with which to study the hydrological effect of tunnels. The application of current hydrological models in karst areas usually requires a large amount of modelling data, especially hydrogeological data that are much more difficult to obtain than those in non-karst areas. To overcome the difficulty of modelling in karst areas, this study presents a new fully physically based distributed karst tunnel hydrological model (KTHM) with a simple structure and simple parameters. The underground confluence module is divided into only two layers: confluence in the epikarst zone and the underground river. Such a simple structure makes it possible to build a model in karst basins with only a small amount of hydrogeological data. In the structure of this KTHM, the tunnel is incorporated in a specially designed submodule to quantify the influence of the tunnel hydrological effect on the water volume. If the tunnel submodule is turned off, then this model becomes a karst hydrological model (KHM) and can be used in other karst basins without tunnels. The simulation results of 20 flow processes and 2 annual runoffs modelled by the KHM and KTHM are compared; the KTHM is more accurate in the study area, which indicates that the tunnel submodule in the model is necessary and that the KTHM is feasible for performing runoff simulations in the study area. The sensitivity sequence of hydrological effects of tunnels on runoff in karst areas is as follows: dry season runoff volume > normal runoff volume > interannual runoff volume > flood peak flow > flood volume.