Abstract: Carbon fiber reinforced polymer (CFRP) has been widely used in externally pasted concrete structures. Magnesium oxychloride cement (MOC) is considered the most potential interfacial bonding material for CFRP−concrete to replace organic adhesive in high−temperature working environments because of its excellent mechanical and high−temperature resistance. In this study, the XRD phase composition analysis was carried out for the optimized modified MOC, and the deterioration of MOC strength after the high temperature was explained microscopically. Then, the interfacial bond performance after high−temperature treatment was analyzed through the single−lap shear test of the interface of CFRP−concrete with MOC adhesive. The test results show that the reinforcing effect of CFRP−concrete structure with modified MOC adhesive is better, and it can keep some strength after high−temperature damage. Finally, in order to solve the problem of low surface strain test data got in CFRP surface strain test with MOC as interface adhesive, the key constitutive parameters of the interface were modified by finite element model modification, and the model formulas of three key parameters were obtained with curve fitting, considering the effect of high-temperature deterioration. The finite element model and the deterioration formula of interface constitutive relation in this study can be used to simulate and calculate the interfacial bond performance of CFRP−concrete interface bonded with MOC in high−temperature environments.