Abstract: The circumstances of the six collapses occurring during the improvement of the concrete cutoff wall in the foundation of the auxiliary dam of Huangbizhuang Reservoir are briefly presented. Variation of the dam foundation composition before and after the collapses and collapse processing are analyzed taking the sixth collapse, the most serious, as an example. Results show that there are solution caverns and submerged current in the bed rock, causing unstability in the bottom of the slot of the cutoff wall, mass loss of the jointing and plugging materials, and liquefaction of the fine size sand layer of the foundation, and finally leading to the collapse of the auxiliary dam collapsed.