Abstract: Soil-caves collapse in karst area is a common geological hazard, which make some disaster accidents. However, the critical condition of forming soil-cave is seldom researched now. Based on the theories of pipe flow, this paper derives the formulas of the critical hydraulic condition for assessing soil-caves. The cohesive soils from Wuxuan county, Guangxi province, China, are chosen to be tested to obtain the critical value by a designed test method. Combining the critical value and the monitoring hydraulic data, the results show the current hydraulic condition will not form soil-caves.