In this paper, the (1-4x)(Na0.5Bi0.5)TiO3-3x(K0.5Bi0.5)TiO3-xBaTiO3 ((1-4x)NBT-3xKBT-xBT) ceramics with x = 0.020, 0.034, 0.040, and 0.050 were synthesized by Sol–Gel flame synthetic approach. Decreased ferroelectric-to-relaxor transition temperature and abrupt decrease of piezoelectric constant reveal that the long-range ferroelectric order would be destroyed when x ≥ 0.040, and a composition boundary between ferroelectric (FE) and relaxor phase (RE) is constructed around x = 0.040. As a result, a large strain of 0.30% under a significantly lower external field of 50 kV/cm was achieved for the critical composition at x = 0.040 due to a reversible RE/FE phase transformation, resulting in a large normalized strains Smax/Emax of 600 pm/V accordingly. Meanwhile, high electrostrictive coefficient of 0.0311 m4/C2 with relatively large strain response of 0.22% was obtained at x = 0.050. The findings may pave a way for obtaining large strain under low electric field and high electrostrictive effect in NBT-based system through the appropriate compositional design and optimized preparation technology.