This paper mainly investigates the formation and evolution mechanisms of microannuli at the casing-cement-formation interface under the combined effects of internal pressure and temperature variations. By taking temperature effects into account, an elastoplastic mechanical model suitable for the casing-cement-formation assembly is established. The model is validated through the Jackson experiment and numerical simulations. The results show that higher temperature and pressure during the loading stage intensify the plastic deformation of the cement sheath, thereby laying the groundwork for interface microannuli to form in the subsequent unloading phase. During the unloading stage, when internal pressure or temperature decreases, tensile stress arises at the interface, directly triggering microannulus formation. In addition, temperature variation significantly influences the size and evolution of these microannuli; under the same unloading pressure, a larger temperature difference leads to a wider microannulus, highlighting the critical role of temperature in interface separation. The study also reveals that the tensile stress at the first interface typically exceeds that at the second interface during unloading, making the first interface more prone to microannulus formation. In particular, a substantial temperature drop during unloading further intensifies relative displacement at the interface, leading to the expansion of microannuli. Overall, the proposed model effectively predicts the risk of interface sealing failure during cementing and production operations, providing vital theoretical and technical support for wellbore integrity assessments and offering valuable engineering applications for enhancing long-term wellbore safety.