Superficial karsts typically develop in the arch of tunnels. Due to the complexity and uncertainty of filling medium, tunnel collapse and roof fall frequently occur. This paper analyzed the instability and mechanical characteristics of the Haide tunnel, which is a superficial karst tunnel with filling medium. The effect of water content on the physical and mechanical properties of filled red clay was analyzed by direct shear test. The pro- cess of instability was analyzed by numerical simulation. Based on the above analysis, the excavation method called three-step and reserved core soil was put forward to treat the karst cave, paired with “Advanced Shed- pipe Support + Advanced Small Pipe Grouting”. Then, the detailed construction technical measures were put forward based on the construction site. A monitoring cross section was added to the specific treatment section to judge the treatment effect to the roof fall section by displacement and surrounding rock pressure. The research results show that the comprehensive treatment plan of the filling karst cave is of good effect, and is referential for similar projects to some extent.