Abstract: The debonding behavior at the interface between FRP sheet and concrete is a key problem for the application of FRP in concrete structure strengthening. The difficulties of finite element analysis of this interface problem are discussed and suggestions are given for the FEA modeling. From the numerical tests with FEA software, the requirements for the constitutive relationship model of concrete are discussed. Based on the analysis, an improved planar concrete constitutive relationship mode is developed and the interface debonding between FRP sheet and concrete under shear force is simulated. The numerical results are found consistent with the test results. The parameter values in the model are suggested. Based on the numerical results, the mechanism of interface is discussed.