The simplified analysis method based on the static equilibrium is generally adopted for raft design. The secondary stress of superstructure due to the differential settlement of the foundation is neglected, which leads to larger support moments and longitudinal bending of raft compared with real values. The spring constitutive relation of composite foundation is obtained by the flat plate loading tests in Karst region. The interaction between the spring and the raft is equivalent to the interaction between the composite foundation and the raft. The model for superstructure-raft-composite foundation interaction analysis is thus established and the raft is designed. This method not only considers the nonlinear properties of composite foundation but also analyzes the influence of superstructure on bending moment and deformation of raft. Compared with the inverted floor method, the calculated values of moment become more reasonable and uneven settlements are considered. This can be references to the design of raft foundation in similar regions. The simplified analysis method based on the static equilibrium is generally adopted for raft design. The secondary stress of superstructure due to the differential settlement of the foundation is neglected, which leads to larger support moments and longitudinal bending of raft compared with real values. The spring constitutive relation of composite foundation is obtained by the flat plate loading tests in Karst region. The interaction between the spring and the raft is equivalent to the interaction between the composite foundation and the raft. The model for superstructure-raft-composite foundation interaction analysis is thus established and the raft is designed. This method not only considers the nonlinear properties of composite foundation but also analyzes the influence of superstructure on bending moment and deformation of raft. Compared with the inverted floor method, the calculated values ​​of moment become more reasonable and uneven settlements are considered. This can be referen ces to the design of raft foundation in similar regions.