Abstract: The dam area of Pingzhai reservoir is dominated with carbonate rocks. Karst develops seriously. A large filled karst cave is discovered in bottom grouting gallery on the left bank of dam site. It is the largest in scale and it mostly affects the seepage of reservoir. The design and construction about the cave are the most difficult one among all of the caves discovered during the excavation of underground engineering. This paper is based on the engineering geological exploration carried on before design stage and the study about regional landforms, geological structure, stratum lithology and river system of surface and underground water. It further combines the karst hydro geological phenomenon and cave fillings discovered during the construction of curtain grouting engineering. This paper examines the four aspects including soluble rock, groundwater sources and transport and erosion of underground river, collapse of cave roof. It studies the formation mechanism of karst development, carries on the analysis. It finds that there are the NW longitudinal tension fractures and horizontal tensile cracks on Jichang anticline and interlayer gliding, and the fault structural fracture zone. They provide the original space and channel for karst system of Wuli Cave-Jichang depression-Shangba depression runoff to Shanchahe. They provide the formation of karst caves of groundwater, the long-term corrosion caused fissure channel to wide crack then to karst pipe, and forms the inverted siphon circulation of karst water at 0+220m0+238m of mileage in the bottom grouting gallery on the left bank. Strong erosion and roof collapse are also immensely helpful in cave formation. Due to deep runoff conditions are changed and channel for karst gradually is filled, the large filled karst cave is developed finally.