The lithology of Dongzhuang Reservoir dam site area is mainly tuff, and the karst seepage problem is more developed in the area. This paper relies on the excavation process of curtain grouting hole WMR4 in the dam site area, encountered a giant soil-filled cavern, carried out supplementary exploration on the scope of the cavern, conducted particle test, compression test, consolidation test and shear test on the cavern-filled clay, and thus put forward the corresponding temporary reinforcing measures, and carried out deformation monitoring on the various monitoring cross-section. The results of the study show that: the cavern is locally developed, with the overall pattern of large upstream and small downstream, and the filling material is high liquid limit clay with high strength; the temporary reinforcement scheme of overrunning circular conduit + steel arch + foundation replacement is adopted, and the monitoring data show that the convergence value of the tunnel tends to be stabilized. This study has important guiding significance for the same type of tunnel.