Abstract: The control rod drive mechanism (CRDM) relies on forced cooling measure to maintain its operating temperature. For the complex issue of CRDM heat transfer mechanism, the thermo-siphoning natural heat convection analysis model was established based on the hypothesis which divides coolant inside CRDM into hot side and cold side, meanwhile, the experiment to measure CRDM inside/outside temperature distribution and total heat dissipating capacity under different conditions was conducted. Through comparison of analysis results and experimental data, the analysis model based on hypothesis can correctly simulate CRDM heat transfer in real situation, the thermo siphoning heat and mass transfer is the major way of CRDM axial heat transfer, and the use of thermal sleeve can weaken thermo-siphoning and reduce heat dissipating capacity effectively.