Abstract: Based on the high temperature heat pipe technology, the transient heat transfer behavior of unit cell on improved thermal-radiator of TOPAZ-Ⅱ proposed by former USSR was analyzed. Different numerical methods, such as finite element method (FEM) for heat pipe wall and wick region and finite difference method (FDM) for heat pipe fin, were employed to simulate the startup process of the heat transfer unit. The strong coupled heat and mass transfer among each heat pipe region was considered based on one-way coupling strategy. In addition, the sonic limit, capillary limit and entrainment limit from Cotter’s heat pipe limits theory were set as the evaluation criteria of heat pipe successful operation. Numerical results show that heat pipe starts up rapidly within 390 s and encounters the sonic limit during stage 2. The steady state of heat transfer unit is finally reached with high isothermality, and the equivalent heat transfer coefficient is 1783.25 W/m². Overall, the heat transfer unit has a good heat removal performance.