Abstract: The frozen startup of alkali metal heat pipes is critical to the safe and stable operation of heat pipe-cooled nuclear reactors. This process involves complex multiphysics interactions, including working fluid melting, liquid-vapor phase change, flow and heat transfer, and structural thermal deformation. This study aims to numerically characterize the coupled thermal-hydraulic-mechanical behavior during the frozen startup of sodium heat pipes, emphasizing transient evolution and the influence of heating power. A two-dimensional axisymmetric model was developed by integrating a self-developed finite element program with COMSOL. The flow and heat transfer in the vapor region and the thermal expansion of the wall were simulated using COMSOL, while the equivalent thermal conduction in the wick and the solid heat conduction in the wall were computed by the dedicated finite element code. The model accounts for phase change at the liquid-vapor interface, temperature-dependent thermophysical properties of materials, and vapor compressibility during the early stages of continuous flow. Validation against experimental data reveals maximum relative errors of 5.62% for the transient process and 1.89% for steady-state conditions, indicating excellent model fidelity and predictive capability. Simulations successfully capture the three characteristic stages of frozen startup. The first stage is characterized by rapid wall temperature rise, creating large axial temperature gradients. The second stage features significant axial pressure and temperature drops induced by vapor compressibility. The heat pipe demonstrates excellent iso-thermal properties at its rated power of 1 000 W in the third stage, evidenced by a steady-state vapor temperature drop of just 2.4 K and a three-step axial wall temperature distribution. Under the constraint of zero axial displacement at both ends of sodium heat pipes, the wall deformation evolves through three successive phases, namely the axial-dominated, transitional, and radial-dominated stages. In the early stage of startup, steep axial temperature gradients drive significant axial displacement. As the temperature gradients gradually decrease, radial deformation begins to grow and eventually dominates. A key observation is that the peak transient total wall deformation during startup is higher than the final steady-state deformation. When the heating power increases from 613 W to 1 200 W, the startup time is shortened by approximately 23.3%. At steady-state, the cross-sectional average vapor velocity decreases by about 81.5%, and the operating temperature increases by approximately 145.9 K, while the maximum wall deformation increases by about 28.9%. This study provides a valuable reference for analyzing and assessing heat pipe frozen startup incorporating structural mechanics effects, aiding optimized startup protocols and enhanced reactor safety.