Abstract: Technetium molybdenum isotope is widely used in the field of nuclear medicine, and its demand accounts for about 80% of the world medical isotope demand. Irradiation in reactor is an important method for production of technetium molybdenum isotope, which is mainly produced and supplied by foreign reactors at present. Therefore, it is of great significance to realize independent production of technetium molybdenum isotope in China. Prior to the radiation study of technetium molybdenum target in reactor, an outside reactor heat transfer experiment should be conducted to verify the rationality of the irradiation device design and obtain its heat transfer characteristics, so as to ensure that the radiation of technetium molybdenum target in reactor will not endanger the safety of the target and reactor. At present, most of the studies on irradiation in reactors are not equipped with forced cooling conditions, and the design of radiation testing devices for natural cooling is rarely carried out. And there are few outside reactor verification experiments for irradiation devices. If necessary outside reactor heat transfer experiments can be carried out, it will provide important technical support for the safety review and subsequent design optimization of irradiation devices. The nuclear heat release generated by the target during irradiation in the reactor test needs to be dissipated by natural convection heat transfer. In order to verify the heat dissipation capacity of the target irradiation device, it is necessary to carry out the heat transfer verification test outside the reactor. In this paper, a natural cooling of technetium molybdenum target irradiation device was designed, and an outside reactor heat transfer test platform was designed to carry out an outside reactor heat transfer verification experiment. The temperature distribution of the experimental section under different heat fluxes, different simulated target diameters and different irradiation device structures was compared and analyzed. The analysis shows that the experiment covers the maximum heat flux of the actual situation inside the reactor, and there is no underheated boiling in this power range, which proves the safety of the designed irradiation device. The design of the diversion tube in the irradiation device can delay the underheated boiling point to 1.1 times of the original thermal power and improve the natural circulation capacity of the device. The increasing of ambient temperature in a certain range is conducive to the enhancement of natural convection heat transfer ability.