Pile holes are often drilled through cavities in karst areas，which induces mud leakage and causes hole wall instability，ground collapse，or equipment trapping and hinders smooth construction.It is important to know the collapse mechanism of boreholes due to mud leakage，so as to optimize and determine the construction process of pile holes.In this paper，indoor model tests were used to study the deformation mechanism and damage mode of soil caused by drilling through cavities and mud leakage in pile holes under the conditions of upper clay and lower sandy soil in the karst area，so as to provide an experimental basis for the decision of pile construction plan in karst area.The results show that the mechanical principle of the pile hole drilling through the roof of the cavity and causing the collapse of the upper layer of clay and the lower sand covering layer is that after the pile hole drills through the roof of the karst cavity，the mud quickly goes down and causes the stress of the hole wall to lose balance，and the sand first moves and fills the pile hole due to the single grain structure，which causes the instability and sinking of the upper layer of clay.When the angle between the top surface of the sand and the horizontal surface is about equal to the angle of internal friction，the sand sliding stops.After the sand sliding stops，the upper layer of clay sinks continuously and forms the soil arch.After the soil arch is formed，the ground tends to be stable.The process of ground sinking induced by drilling mud channeling can be divided into three stages：rapid sinking，slow sinking，and stabilization.The whole process takes about one minute.The ground forms a sinking basin.In the project，it is suggested to reinforce the sand on the top of the roof of the karst cavity by grouting to prevent the ground collapse caused by the pile hole drilling through the roof of the karst cavity，and then the safety of the equipment or personnel and smooth construction can be ensured.