Abstract: The nuclear LT21 aluminum alloy was used as research object, and 1.2MeVSi⁵⁺ was used to carry out the irradiation experiment on it.By SEM, TEM and nanoindentation, the changes in the structure morphology and mechanical properties of the irradiated material were analyzed.The results show that after 1.2MeVSi⁵⁺ irradiation, the secondary phase precipitates in the aluminum alloy, and the secondary phase is mainly composed of silicon and iron, and the sample has a clear tendency to harden after irradiation.LT21 alloy is an important structural material used in 101 reactor.The study of its changes after ion irradiation is beneficial to the development and progress of the 101 reactor decommissioning work, which can provide a comparison for the subsequent research on the aluminum alloy after irradiation in reactor.