In order to better understand thermal aging mechanism of reactor pressure vessel steels with the action of dislocation loop, the hardening behavior and microstructure evolution of Fe-10at. %Cu-3at. %Mn-1.5at. %Ni-1.5at. %Al alloy is investigated using phase-field method. The results showed that the precipitation includes spinodal decomposition of Cu-rich precipitates (Cu/Ni (Al, Mn) core–shell structure) assisted by dislocation loops, spinodal decomposition assisted by new dislocations, and nucleation and growth induced by thermal aging. Ultimately, a gradient of Cu-rich nanoprecipitates is formed, which can result in high strength-ductility synergy. Phase-field method provides better details for predicting the morphology, distribution, and size of Cu-rich precipitates in Fe-Cu-Mn-Ni-Al alloy with dislocation loops. The simulation results are introduced into the hardening model of copper-rich precipitates to achieve a closed-loop of process → microstructure → performance.