The existing karst caves below the floors of the karst tunnels have direct influence on the tunnel construction and railway operation. Therefore, the analysis and assessment on the stability of the floors of the karst tunnels is one of the critical issues for the karst tunnel construction. In the paper, a semiquantitative evaluation method is provided for the minimum safe thickness of the floors of the karst tunnels by means of the load transferring line concept; theoretical formulas are established for the minimum safe thickness of the floor rocks of the  karst tunnels based on the criteria of tension strength and shear strength of rocks by means of elastic theories. The paper can provide reference for the assessment on the safety of the floors of the karst tunnels during construction.