A suitable model for high-temperature heat pipe startup is a prerequisite to realizing the numerical simulation for the heat pipe cooled reactor startup from the cold state. It is required that this model not only describes the transient behavior during the startup period, but also reduces the computing resources of the heat pipe cooled reactor simulation in the simplest way. In this study, a simplified model that integrates the two-zone and network models is proposed. In this model, vapor flow in the vapor space, evaporation, and condensation in the vapor–liquid interface are decoupled with heat conduction to achieve a fast calculation of the transient characteristics of the heat pipe. An experimental system for a high-temperature heat pipe was developed to validate the proposed model. A potassium heat pipe was utilized as the experimental material. Startup experiments were performed with different heating powers. Compared with the experimental results, the accuracy of the proposed model was verified.Moreover, the proposed model can predict the vapor flow,pressure drop, and temperature drop in the vapor space. As indicated by the analysis results, the essential requirements for successful startup are also determined. The heat pipe cannot achieve a successful startup until the heating power satisfies these requirements. All the discussions indicate the capability of the proposed model for the simulation of a high-temperature heat pipe startup from the frozen state;hence, can act as a basic tool for the heat pipe cooled reactor simulation.