During concrete cutoff wall construction of Huangbizh uang reservoir secondary dam reinforcement project, seven large -scale ground collapses occurred. This kind of collapse, rarely happened all over the world, is quite different from normal slot collapses during concrete cutoff wall construction, typically caused by mass loss of partia l strata material. By investigating the features of ground collapses, the paper summarizes three elementary factors and two elementary procedures of ground collapses formation. By employing mechanical analytical methods, the mechanism is analyzed that a caving hole would be formed due to the drain of sands in sand-bed. The process of sand-bed progressively caving is discussed also. By means of large deformation numerical method, the pr ocess of ground collapse caused by caving hole in sand-bed, which is set before computation, can be simulated.