<p>Water inrush disaster has been one of the major disasters in the process of tunnel construction in karst areas. To study the water inrush disaster forming mechanism of filled karst conduit with different types of filling materials, a simplified model was proposed, and a series of large-scale similar model tests have been carried out with a self-developed 3D model test system, which can apply stable water pressure. The influence of hydraulic pressure on water inrush forming process in karst conduits with different filling materials was investigated. Test results show that the particles of the filling materials gradually lost from the free surface and the water inrush channel gradually generated and expanded. The eventual water inrush channels could be divided into two categories based on different filling materials: conduit controlled channel and filling material controlled channel. The critical hydraulic pressure required to form a water inrush channel is lower for the former one than that for the latter. Based on the instability characteristics of filling material particles, a local instability mechanical model was established by deriving a mathematical formula of critical hydraulic pressure when water inrush occurs. The mechanical model reveals the mechanism of water inrush to an extent. The calculated critical hydraulic pressures for different types of filling materials were in good agreement with the test results. The research might provide guidance for predicting water inrush disaster during tunnel construction in karst areas.</p>