Abstract: A two-dimensional transient numerical model with interphase coupling inside a cylindrical heat pipe is established to investigate the flow and heat transfer characteristics of a horizontal trapezoidal groove-mesh composite wick heat pipe. A porous medium model combined with a user-defined function (UDF) is adopted to simulate the gas-liquid flow and heat transfer processes within the composite wick. The results show that compared with the single trapezoidal groove wick, the composite wick changes the flow pattern of the working fluid due to the variation of the flow cross-sectional area. The liquid velocity and pressure drop decrease with the widening of the wick channel, while the vapor velocity and pressure drop increase as a result of the contraction of the vapor chamber channel. Although a high-density mesh screen helps reduce thermal resistance owing to its high thermal conductivity, its low permeability aggravates the flow loss and thus weakens the backflow capacity of the working fluid. In addition, increasing the heat flux in the evaporator section enhances heat transfer, but also induces a significant gas-liquid pressure drop gradient, which further limits the backflow driving force.