To study the heat transfer mechanism of lithium heat pipes and to improve their application in small reactors, a solid-liquid-gas three-phase coupled model of the tube wall, the wick and the vapor chamber inside the tube is developed using COMSOL Multiphysics software. The results show that when the heat flow in the evaporation section increases from 13.9 kW to 20.8 kW, the pipe wall temperature, steam temperature, steam pressure, liquid pressure inside the wick, and liquid axial velocity increase with the increase of heating power, while the steam axial velocity first increases and then decreases with the increase of heating power. In the steady-state operation, the pipe wall temperature decreases step by step, while the vapor temperature and pressure remain basically unchanged, indicating that the lithium heat pipe has good isothermality. © 2022 Yuan Zi Neng Chuban She. All rights reserved.