Abstract: Heat pipe cooled reactor has the advantages of simple structure, expandability, safety and reliability, and is one of the ideal choices for space and deep sea nuclear power. However, the thermal expansion effect of the heat pipe inside the core when operating under high temperature conditions can cause the heat pipe to deform, which affects the heat transfer capability and operating stability of the heat pipe. Therefore, it is important to study the flow, heat transfer and mechanical properties of the high temperature alkali metal heat pipe for the safety of heat pipe cooled reactor. In this work, a multiphysics coupled model of the high temperature heat pipe was constructed using COMSOL Multiphysics finite element software. The steady-state, weakly compressible laminar flow model was used for the gas part, and the porous medium model based on Darcy's law was used for the region of wick. The flow heat transfer characteristics of the working medium inside the heat pipe and the thermal expansion effect of the pipe wall were studied by numerical simulation. The results show that the relative error between the calculated results and the experimental values of the model is less than 1%, the flow heat transfer characteristics of the high temperature heat pipe can be accurately simulated by the model in this paper. The model can reveal the heat and mass transfer mechanism inside the alkali metal heat pipe more comprehensively and obtain the pressure and velocity information inside the heat pipe that can't be measured experimentally. The pressure gradient and temperature gradient along the axial direction during the operation of the heat pipe are small and have good isothermal properties. The high temperature of the heat pipe operation will cause the wall to expand, and there is a maximum 1.75% total deformation compared to the cold state at rated power. The variation of the transmission power can significantly affect the distribution characteristics of the key parameters of the working medium inside the heat pipe. The increase of power leads to the decrease of the vapor flow rate and the increase of the liquid flow rate inside the heat pipe, which raises the operation temperature and increases the deformation of the wall at the same time. During engineering fabrication and assembly, improper control of heat pipe deformation may result in excessive stress values at the heat pipe and reactor core assembly connection, and the effect of heat pipe deformation needs to be considered to improve the operational stability of the heat pipe reactor.