Abstract: Heat pipes are heat transfer components that rely on the phase change of their internal working fluid to achieve heat transfer. When using alkali metals such as lithium, sodium, and potassium as the working fluid for heat pipes, high temperature heat pipes with operating temperatures exceeding 750 K can be developed. High temperature heat pipes offer advantages such as strong heat transfer capability and excellent temperature uniformity, making them highly valuable in the field of heat pipe reactors and serving as the core heat transfer components in heat pipe reactor systems. The start-up of high temperature heat pipes involves transitioning from a frozen state to the establishment of a continuous vapor flow internally and reaching the final steady state. This process involves multiple complex heat transfer and mass transfer mechanisms. The development of an operational analysis model that balances computational efficiency with accuracy, coupled with the investigation of operational characteristics for reactor-grade heat pipes, is crucial for conducting comprehensive system-level analyses of heat pipe reactor. In this study, a improved thermal resistance network model based on the thermal resistance network and flat-front model was constructed. The model simplified the complex heat transfer and mass transfer processes within the heat pipe based on reasonable assumptions. It addresses heat conduction through the pipe wall, heat transfer in the wick structure, and vapor phase change along with axial heat transfer. It enables transient simulation calculations for high temperature heat pipe operations. The computational results were compared with experimental data from self-developed sodium heat pipes, revealing that the calculation relative deviations during the start-up transient process are less than 15.6%, and those under steady-state conditions are less than 2.3%. The model’s validity and accuracy were confirmed, demonstrating both satisfactory computational speed and precision. Systematic investigations were conducted on the operation characteristics of the sodium heat pipe under different working conditions. The computational results align with the experimental findings. Additionally, the model was extended to compute parameters not covered by the experiments. Key parameters such as heat flux, heat transfer coefficient, length ratio and structural dimensions were analyzed. The results show that prior to reaching the heat transfer limit, increasing the heat flux in the evaporator accelerates heat pipe start-up, elevates steady-state temperature and reduces thermal resistance. In contrast, under constant heat flux condition, when structural or operational conditions enhance cooling capacity, the start-up time increases, accompanied by a decline in the steady-state temperature and an increase in thermal resistance. Furthermore, the critical start-up power of the heat pipe exhibits a linear relationship with the heat transfer coefficient and the length of the condenser.