Abstract: Pre-applied annulus back pressure cementing can effectively improve the sealing ability of cement sheaths and reduce the occurrence of annulus pressure buildup. In order to accurately understand the mechanical mechanism, the model for calculating the stress on the casing-cement sheath interface was developed based on elastic mechanics and continuity condition of interfacial displacement with consideration of the operation process of pre-applied annulus back pressure cementing. With the model, the mechanical mechanism for improving sealing ability of the system of double layer casing-cement sheath with pre-applied annulus back pressure cementing was further studied. The accuracy of the theoretical model was verified by experiments. The influence of various factors on the sealing ability of the interface was studied by the mechanical model proposed. The results showed that increasing the pre-applied annulus back pressure during cementing could significantly increase the radial stress on interface, resulting in strengthening the sealing ability of cement sheath. The radial stress on interface increases linearly with the increase of annulus pressure. The pre-applied annulus back pressure cementing works better on the improvement of sealing ability of the inner casing-cement sheath interface. A relationship was delineated: the greater the elastic modulus of the cement sheath, the greater the radial stress on the interface, and the stronger the sealing ability. The results indicated that the pre-applied annulus back pressure cementing could significantly increase sealing ability of the interface, and the model has provided a quantitative calculation method for the radial stress on the interface. The research finding is of great importance to the design and operation of the pre-applied annulus back pressure cementing in oil and gas wells.