The present invention relates generally to an apparatus of assembling a capsule for neutron-irradiation experiments. The capsule may contain test specimens to develop nuclear fuel, and may comprise a capsule main body and protection tube that may be assembled and disassembled using separate joining means. The present invention provides a capsule assembling apparatus that may enable safe and easy assembly/disassembly of the capsule placed at a given depth in a working pool through remote working. The capsule assembling apparatus may comprise a base structure loading the capsule main body, a guiding pipe of a given length extending from the base structure to provide a pathway through which the capsule main body is loaded on the base structure, a damper fixing the capsule main body to the guiding pipe, and a coupler coupling the capsule main body with the protection tube.