Abstract: After arch closure and subsequent operation, extra-high arch dams generally exhibit temperature recovery. The internal mechanism underlying this phenomenon, its patterns, and its potential impacts on the long-term structural safety of the dam have emerged as a key concern in the field of hydraulic engineering, given the critical role of such large-scale structures in water conservancy and hydropower projects. To thoroughly clarify the intrinsic mechanism of temperature recovery during the operational phase of extra-high arch dams, characterize its temporal and spatial variation laws, and quantify its effects on the dam’s structural performance—thereby providing robust technical support for ensuring their long-term safe, reliable, and sustainable operation—this paper conducts a systematic and in-depth study with a specific extra-high arch dam as the engineering case study. This research integrates multiple complementary and advanced research methods, including prototype monitoring, regression analysis, and refined numerical simulation, to achieve a comprehensive and accurate understanding of the temperature recovery process. Firstly, based on an array of monitoring instruments strategically embedded at key locations within the dam body—such as precision thermometers for tracking temperature changes, stress-free gauges for measuring unconstrained thermal deformation, and plumb lines for monitoring structural displacement—long-term, continuous, and high-quality measured data related to temperature and structural response are collected. Secondly, regression analysis is employed for parameter inversion, a technique that enables the accurate determination of the late-stage adiabatic temperature rise model of concrete and its key thermodynamic parameters (e.g., thermal conductivity, specific heat capacity) by fitting the measured data. Finally, a high-precision three-dimensional finite element model is established, which is capable of simulating the entire life cycle of the dam, spanning from the construction phase (including concrete pouring, curing, and arch closure) to the long-term operational phase. Through thermo-mechanical coupling analysis, this model quantitatively reveals the underlying mechanism of temperature recovery and the corresponding structural response of the dam under thermal loads. The study yields the following key findings and results: (1) The dominant factor driving temperature recovery is identified: the residual hydration heat of concrete within the dam body is the fundamental cause of the late-stage temperature rise. This residual heat is continuously released during the prolonged hydration process of concrete, even after the completion of arch closure. In contrast, heat transfer from the external environment (including ground temperature, ambient air temperature, and reservoir water temperature) into the dam body—referred to as “heat backflow”—primarily affects the near-boundary regions of the dam. Due to the large volume and low thermal conductivity of the dam concrete, this external heat has a negligible impact on the central core area of the dam, and any observable effect is characterized by a significant time lag. (2) The spatiotemporal evolution of the dam’s temperature field is accurately characterized with high resolution: following arch closure, the dam’s temperature continues to rise at a gradually decreasing rate, reaching its maximum value after approximately 3 to 6 years. Subsequently, the temperature begins to decrease slowly as the hydration process weakens and thermal equilibrium with the surrounding environment is gradually established. The entire process of temperature rise and stabilization takes about 20 years to reach a long-term stable temperature field consistent with the thermal conditions of the surrounding environment. (3) The influence mechanism of temperature recovery on the dam’s structural response is clearly elucidated: the thermal load induced by temperature rise leads to thermal expansion of the concrete, resulting in a noticeable upstream bulging deformation in the mid-section of the dam—a typical response to uneven thermal stress distribution. In terms of stress distribution, significant transverse and vertical tensile stresses generally occur on the upstream surface of the dam due to the constraint of thermal expansion, while compressive stresses are prevalent on the downstream surface, which is conducive to the structural stability of the arch dam. Although in localized stress concentration areas, such as the lower part of the middle discharge holes, the tensile stress can reach 4 to 5 MPa under the combined action of thermal load and structural constraints, detailed calculations confirm that this stress level is well within the design limit of the concrete’s tensile strength specified in relevant engineering codes and standards. Additionally, the overall compressive stress level of the dam body has increased slightly due to thermal contraction after the peak temperature, with the maximum compressive stress measuring 5 to 6 MPa—far below the ultimate compressive strength of the concrete material, indicating a sufficient safety margin against compressive failure. Temperature recovery of extra-high arch dams during their operational phase is an inherent, long-term physical process dominated by the material properties of concrete, and its potential impacts on dam performance cannot be ignored in engineering design, construction, and operation. The calculation model established in this paper can reliably reproduce the temporal and spatial evolution of temperature during the dam’s operation period, and the systematically revealed temperature rise mechanism and structural response characteristics provide a robust scientific basis for optimizing temperature control measures during the late operational stage, enhancing the efficiency and accuracy of safety monitoring systems, and formulating rational, targeted maintenance and reinforcement strategies for extra-high arch dams to ensure their long-term structural integrity and operational safety.