Abstract: Aiming at the temperature rising in the secondary side of the passive residual heat removal heat exchanger (PRHR HX) at initial operating stage, experiments on the heat transfer of vertical tube bundle immerged in an elevated tank during the heating up period were performed. The results show that in the early stage of experiments, heat is transfered by single-phase natural convection due to the large subcooling of water. The water temperature rises gradually which leads to a thermal stratification phenomenon in the tank and the heat transfer capability decreases with the water temperature increasing. As the subcooling decreasing, the heat transfer mechanism transforms from single-phase convection to subcooling boiling gradually. After water reaches the saturation temperature, saturated pool boiling is the primary mechanism of heat transfer. Adopting Churchill & Chu correlation, the natural convection heat transfer coefficient was separated from the total heat transfer coefficient. The proportion of single-phase natural convection and subcooling boiling at different heat transfer stages was analyzed. This work provides certain directive significance to the design of PRHR HX.