In order to clarify the stress variation law of casing in salt-paste stratum and provide theoretical basis for wellbore structure design, the model of salt-gypsum stratum and casing-cement sheath-stratum was established, and the stratum stress distribution and bearing capacity of the casing were studied by numerical simulation method. The effect of different defect types of the cement sheath (ring opening, depth and direction) on the bearing capacity of casing were emphatic discussed. The results show that the overall error of stress simulation results was small, and this model can be used to predict the original in-situ stress distribution of the stratum. The in-situ stress acts on the cement sheath through the stratum, and then imposed on the casing. The stress acting on the casing is the largest, and the casing is most prone to deformation under the action of non-uniform in-situ stress. When the annular opening of the cement sheath is about 66°, the stress on the casing is maximum. With the increasing of the defect depth of cement sheath, the stress acting on the casing increased. Under the condition of the same defect depth, the stress caused by the defect from the inside-out is obviously higher than that caused by the defect from the outside-in direction.