Chemical doping is favored by academia as well as industry because of its effectiveness in attuning to the properties of piezoceramics. Although significant progress has been made, few reports have focused on the role and overall effect of substituted ions. Based on the tendency of special crystals such as ZnO toward spontaneous growth, this study applies the concept of composite modulation to conventional doping; the CuO-modified 0.2Pb(Zn1/3Nb2/3)O-3-0.8Pb(Zr1/2Ti1/2)O-3(PZN-PZT) system has been used for verification of the proposed method. The results show that copper ions enter the perovskite matrix to specifically replace the zinc ions causing lattice distortion and increasing the rhombohedral phase (RP) content. Furthermore, the substituted zinc ions enter the grain boundaries and grow into a secondary phase ZnO, based on their spontaneous-growth tendency; the induced heterogeneous interfacial effects lead to refinement of the domain size and enhancement of the interface polarization. The combined effects of the lattice substitution and composite modulation promote a significant improvement in the piezoelectric coefficient of the CuO-modified PZN-PZT system compared with its pure counterpart. The dual function of doping demonstrated in this study is expected to further contribute to the preparation and performance improvement of the other piezoelectric composites.