Abstract: The formation of Cu-enriched clusters is one of the main factors affecting the safe service of FeCu alloy. Based on the AKMC (atomic kinetic Monte Carlo) method, the effect of alloy element Mn on the formation of Cu-enriched clusters during thermal ageing in the Fe-1.34at.%Cu-Xat.%Mn (X is 0, 0.27, 1.2, 3, and 5) alloy was studied. The research results show that with the increase of Mn content in the alloy, the number density of nano-scale Cu-enriched clusters continues to increase, and Mn content has little effect on the average size of Cu-enriched clusters. The cluster is enriched in Cu, and the Mn content in the cluster increases with the Mn content in the matrix. The percentage of alloy elements involved in the formation of clusters determines the average cluster size. The higher diffusion ability and longer diffusion distance of the Cu-Vac complex are the main factors that cause the precipitation of Cu-enriched clusters, and the addition of Mn will inhibit the diffusion mobility of the Cu-Vac complex, thereby increasing the number density of the cluster. The addition of alloy element Mn will affect the size and number density of Cu-enriched clusters, thereby affecting the hardening effect caused by Cu-enriched clusters.