Abstract: Subsurface soil-voids are main potential geologic hazards being faced in engineering construction in karst regions. In order to analyze the formation mechanism of soil-voids, laboratory tests including physics model experiment and slaking test have been taken based on site investigation. For the regions with single-layer clay or silty clay, the authors divide the process of soil-void development into three phases corresponding to the void’s position and its relationship with groundwater table. In the first phase, the initial soil-void forms usually along the weak zones of soil such as fractures and bedrock surface, and seepage deformation induced by karst water flow plays an important part in this process. In the second phase, the soil-void exists in the belt of phreatic fluctuation, and the soil disintegration for repeat soak is the key reason in the formation of soil-void. In the last phase, the soil-void is generally within the unsaturated zone and closes to ground, so the rainfall and surface water leakage along the soil fracture will influence the stability of the soil-void greatly.