Electrostrictive materials fabricated with relaxor ferroelectrics have attracted much attention because of their excellent features (e.g., high precision displacement). In this study, we realized the dielectric relaxation behavior of bismuth ferrite (BiFeO₃, BFO)-based ferroelectrics by designing polar nanostructures. Accompanied by the strengthened random field and the degenerated octahedral distortion, the grain inhomogeneity featuring the coexistence of strong piezo-response polar cluster grains and weak piezo-response polar entity grains contribute to the relaxation behavior. Finally, such a dielectric relaxation behavior results in a large electrostrictive strain (S = 0.18%–0.27% @ T = 20–100 °C) together with the composition/electric field/temperature independence of Q₃₃. We believe that our research provides a new paradigm to study the dielectric relaxation behavior and optimize electrostrictive properties of perovskite materials.