Based on the theories of fracture mechanics and composite materials mechanics, fibrous composite model that interacted on stratum, cement loop and casing was firstly established. Boundary conditions that the repaired force acted in fibrous composite model were also given. The influences of interface damage evolution and fracture process of fiber and matrix on the cement loop mechanical properties were studied as well. Meanwhile the distributions of stress and displacement fields were obtained. Finally, the debonded length and variation of the optimum repaired force were also determined, which can restrict the cement damage length to the lowest scale. The results given here can provide a reliable design references for improving the technology of repairing casing damage well.