<p>CO₂ geological storage is widely regarded as one of the most technically mature and large-scale options for emissions reduction within CCUS (Carbon Capture, Utilization and Storage) technologies. However, CO₂ corrosion and long-term injection/storage operations pose significant threats to the sealing integrity of cement sheaths in wellbores. Existing studies predominantly rely on numerical simulations to analyze the mechanical response on cement sheath under temperature–pressure conditions, neglecting the corrosion of cement sheath. However, there is a notable lack of theoretical computation models specifically for cement sheaths containing the coupled effects of corrosion and stress. This study establishes a mechanical model of the casing-CCS (corroded cement sheath)-formation assembly based on elastoplastic theory, incorporating thick-walled cylinder theory and the Mohr–Coulomb criterion. The model is solved using MATLAB to quantify the stress-displacement response of CCS during injection and storage. Results indicate that CO₂ corrosion significantly increases the complexity of the stress state in the cement sheath and the risk of sealing failure. For a representative injection pressure of 60 MPa, introducing a 5 mm corroded layer increases the predicted micro-annulus aperture at the primary casing-cement interface from 0.0238 mm to 0.0519 mm after unloading (an increase of 118%). When the injection pressure varies from 40 to 100 MPa, the micro-annulus aperture grows from 0.0211 mm to 0.1113 mm, whereas increasing the CSCL thickness from 5 to 30 mm only enlarges it from 0.0519 mm to 0.0579 mm (11.5%). In contrast, increasing casing wall thickness from 7.5 mm to 17.5 mm reduces the maximum radial stress in the cement sheath from -28.88 MPa to -18.68 MPa and decreases the micro-annulus aperture from 0.1829 mm to 0.0413 mm. This study provides a theoretical foundation for optimizing wellbore design in CCUS projects.</p>