Abstract: Miniature Neutron Source Reactor (MNSR) low enriched uranium (LEU) conversion and decommissioning include the unloading operation of discharging spent fuel, and unloading device is one of the key equipment to ensure operation safety. When the existing unloading device is used, the sealing performance and tightness of MNSR reactor vessel will be destroyed and can’t be restored. However, it is necessary to use the original reactor vessel to recover the reactor normal operation after LEU conversion. On the basis of LEU conversion requirements, the new unloading device was designed, which could be used in the unloading operation without separating reactor vessel and destroying the sealing performance and tightness. The new unloading device consists of unloading operating tools and auxiliary mechanical device. The unloading operating tools can be used to grab the fuel assembly only through the small cover opening of reactor vessel top plate. The auxiliary mechanical device can be used with unloading operating tools to realize operation in a long distance by lifting and level motion. The calculation and analysis results show that the structure strength of unloading device is much larger than the actual load to ensure operation safety and reliability. The unloading device is reasonable and reliable, and can be used in the practical operation of MNSR LEU conversion or decommissioning at home and abroad.