In order to find out the reason of many “tiankeng” in southwest of china in recent years, based on seepage damage theory, the theory of saturated soil seepage and changed water head equipment were used to simulate the change process of water head in geologic norm, considering the influence of vacuum erosion as well. At first, physical property and chemical composition of soil samples were got by in-situ test and laboratory test before destroy test, and the result shows that the surface subsidence is not caused by carbonate karst cave. Then changed water head equipment was designed to simulate the change process of water head automatically. The gradation, water head and sample deformation characteristic of soil before the destroy experiment were compared with those after the destroy experiment. It is show that soil sample chemical property changes little, but little soil particles reduce significantly. With the change of gradation and structure of soil, the bearing capacity decreases and eventually it leads to the soil mass collapse. The changed water head test clarifies the reason and mechanism of “tiankeng” phenomenon reasonably.