High-entropy alloys with BCC and FCC coexisting structures usually have excellent comprehensive mechanical properties, and Al element can promote the transformation of Cu-containing high-entropy alloys from FCC structure to BCC structure to obtain the BCC and FCC coexisting structures. In order to illustrate the process of phase separation of high entropy alloys, a low-cost Al-TM transition group element high-entropy alloy is selected in this work. Based on the Chan-Hilliard equation and Allen-Cahn equation, a three-dimensional phase field model of Al_xCuMnNiFe high-entropy alloy is established, and the microscopic evolution of the nano-Cu-rich phase of Al_xCuMnNiFe high-entropy alloy (x = 0.4, 0.5, 0.6, 0.7) at 823 K isothermal aging is simulated. The results show that the Al_xCuMnNiFe high-entropy alloy generates two complex core-shell structures upon aging: Cu-rich core/B2_s shell and B2_c core/FeMn shell, and it is found through discussion and analysis that the formed B2_c plays an inhibitory role in the formation of the nano-Cu-rich phase, and that this inhibitory role becomes larger with the increase of Al element. Combining the empirical formula, the curve of yield strength of the Cu-rich phase varying with the aging time is obtained for the Al_xCuMnNiFe high-entropy alloy, and the overall yield strength of the high-entropy alloy has a rising-and-then-falling trend with the change of time, and the aging time of the peak yield strength and the alloy system are obtained from the change of the curve, so that the best alloy system and aging time of the high-entropy alloy can provide a reference for aging process.