<span>In-service thermal loading of gear shafts may relax the residual compressive stresses introduced by ultrasonic impact treatment (UIT), thereby degrading the surface strengthening effect. In this study, a 30CrMoA gear shaft is investigated using integrated finite element simulation and experimental approaches to systematically evaluate the effects of UIT on surface residual stress, microhardness, surface roughness, and microstructure, as well as the thermal evolution of residual stress at 65 °C. Results demonstrate that UIT introduced a residual compressive stress field extending to approximately 1.25 mm beneath the surface, with a surface stress of −321 MPa and a peak stress of approximately −724 MPa. Surface hardness increased from 231 HV to 318 HV, with a hardened layer depth of approximately 1.14 mm, while surface roughness Ra is reduced from 0.8 μm to 0.18 μm. Under thermal exposure at 65 °C, surface residual compressive stress exhibited an initially rapid relaxation that subsequently decelerated, stabilizing after 8 h with a total relaxation of approximately 23.94%. Meanwhile, surface hardness, hardened layer depth, and microstructure remained essentially unchanged, indicating excellent retention of the work-hardening effect. Finite element predictions showed excellent agreement with experimental measurements, validating the effectiveness of the numerical model. This study elucidates the decoupled thermal evolution of residual stress relaxation and work-hardening retention in UIT-strengthened surface layers, establishing a theoretical framework for the reliable engineering application of ultrasonic impact treatment to gear shafts operating at elevated temperatures.</span>