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Influence of K/Na ratio in (KxNa1−x)NbO3 on the ferroelectric stability and consequent changes in the electrical properties of 0.99(Bi0.5Na0.4K0.1)TiO3–0.01(KxNa1−x)NbO3 (BNKT–KxNN) ceramics were investigated. Results showed that change of K/Na ratio in KNN induces a phase transition from ferroelectric to ergodic relaxor phase with a significant disruption of the long-range ferroelectric order, and correspondingly adjusts the ferroelectric–relaxor transition point TF−R to room temperature. Accordingly, giant strain of ~0.46% (corresponding to a large signal d33* of ~575 pm/V) which is comparable to that of Pb-based antiferroelectrics is obtained at a K/Na ratio of ~1, and the emergence of large strain response induced by the change of K/Na ratio of KNN can be well explained by the correlation between the position of ferroelectric–ergodic relaxor phase boundary in the BNKT–KxNN system and the tolerance factor t of the end number (KxNN). In situ high-energy X-ray scattering experiments with external field reveals that the large strain response in the studied system is likely related to the electric field-induced distortion from the pseudocubic structure.