Rare earth doping has been widely used to modify electrical properties of piezo/ferroelectric materials. However, the physical mechanism behind property enhancement and the reason of property discrepancy induced by rare earth has been rarely solved in bismuth ferrite-barium titanate (BF-BT) ceramics. Here, two kinds of rare earth ions with different ion radii (La³⁺ and Sm³⁺) were separately introduced into Bi_1-xRe_xFeO₃-BaTiO₃ (Re = La, Sm). As a result, doping an appropriate amount of rare earth ions (La³⁺ x=0.05 and Sm³⁺ x=0.04) can effectively improve the ferroelectricity and strain response. By combining ferroelectric scaling behavior and multi-level structure elucidation, the enhancement of electrical properties after La³⁺/Sm³⁺ doping can be attributed to the increase of total number of activated domains, the increased Bi ions off-centering and displacement of B-site ions coupled with the enhanced lattice deformation. Importantly, stronger lattice shrinkage and BO₆ octahedral distortion can be achieved in Sm-doped ceramics, leading to a stronger intrinsic ferroelectric and strain response than that of La-doped ceramics, although a large enough electric field was required to release the intrinsic response because of the larger random field and dielectric relaxation induced by the larger ionic radius difference between Bi³⁺ and Sm³⁺. Oppositely, the decreased coercive field and promoted domain switching make the electrical response of La-doped ceramics can be released under a much lower electric field. The work highlights the role of rare earth doping and provides a paradigm for further designing high-performance BF-BT ferroelectrics.