Sustained casing pressure is common phenomenon after multistage fractusheath in shale gas Wells, which seriously affects wellbore productivity and life. This study focuses on a specific shale gas well in the geographical area of the Sichuan Basin, where a cement sheath completeness evaluation model was constructed for its vertical well section. Using numerical simulation, the distribution characteristics of the stress field and the failure parameters in accordance with the Mohr - Coulomb criterion for the A - annulus cement sheath were revealed. At the same time, the damage evolution mechanisms of the cement sheath under complex loading circumstances were systematically interpreted. The expansion characteristics of the swelling cement paste during the hardening phase were analyzed using a comparative thermal stress evaluation algorithm, also, research efforts were dedicated to exploring the impact of the swelling features on both the stress state and the completeness of the cement paste. Simulation data indicates progressive elevation of radial stress in the annular cement structure as borehole depth advances, whereas concomitant reductions manifest in both hoop stress magnitudes and Mohr-Coulomb failure criterion parameters. The cement sheath located over the bottom part of the casing is inclined to suffer from circumferential tensile failure, and the cement sheath under the casing shoe is prone to undergo radial compression failure. Decreasing the Young's modulus of the cement sheath and enhancing Poisson's ratio has the capacity to mitigate the stress regime. Nevertheless, it is inadequate to ensure the completeness of the cement sheath in the course of the fracturing operation. Conversely, the expansive cement paste system induces an increment in radial stress, concomitantly causing a diminution in circumferential stress and a reduction in the Mohr-Coulomb decision parameter. In cases where the expansion ratio is low, solely increasing the Young's modulus of the cement sheath is insufficient to guarantee its hermetic completeness. When the expansion ratio is in the appropriate range, the effective hermetic height shows an upward trend as the Young's modulus experiences an increment.