Abstract: Based on the experiment and theoretical studies, the relationship between the collapse of covered karst during underground erosion and the fissure width of the limestone was examined. The results show that the critical fissure width is the key factor for the collapse of covered karst due to underground erosion. In the meanwhile, the types of covered soils, compactness, and critical hydraulic gradient are also important. The assessment method of critical fissure width was set up.