In view of the serious casing damage in the mudstone interlayer and near the fault of complex Dongxin fault block reservoir, the casing damage mechanism is studied. The studies demonstrate that the main causes of the casing damage in the mudstone interlayer and near the fault are the abnormal in-situ stress there. Therefore, considering casing damage caused by the abnormal stress and non-uniform stress in these 2 stratigraphic sections, the optimization design method of casing strength is put forward based on the in-situ stress.