Atom probe tomography characterization and thermal aging method have been employed to investigate the precipitation of Cu-rich clusters in reactor pressure vessel （RPV） model steels with and without the addition of Ni alloying element. It is observed that the number density of Cu-rich clusters in the PRV model steel with the addition of Ni （0.84 wt%） is higher than that without the addition of Ni. The segregation of nickel in the Cu-rich clusters and at the boundaries of clusters/matrix was also observed. It is found that the nickel alloying element can promote the precipitation of Cu-rich clusters in RPV model steel. An analytical embedded atom method model of Fe-Cu and Fe-Cu-Ni system is constructed on the basis of the physical properties of pure constituents Fe, Cu and Ni. The calculation results show that the nickel （1 at%） can promote the precipitation of Cu-rich clusters. The calculation results agree with the results of experiments.