In this study, we designed a series of compositions deep in pseudocubic region based on ternary (1-x)[(1-y)(Bi0.5Na0.5)TiO3-y(Bi0.5K0.5)TiO3]-x(K0.5Na0.5)NbO3 (BNKT100y-100xKNN) system for electrostrictive applications. Results show that the KNN substitution into BNKT100y induces a significant disruption of the ferroelectric order, and tends to enhance the electrostrictive properties of the ceramics. A high, purely electrostrictive effect with large electrostrictive coefficient Q (33) of 0.025 m(4)/C-2 and good thermostability comparable with that of traditional Pb-based electrostrictors is obtained for BNKT100y-xKNN (y = 0.20, x = 0.16) ceramics, which lies deep in pseudocubic region along the MPB compositions of the ternary system. Temperature-dependent structural analysis suggests that the good electrostrictive properties insensitive to temperature can be ascribed to the stable relaxor pseudocubic phase over a wide temperature range. Furthermore, it is found that the electrostrictive coefficient Q (33) exhibits only small degradation within 2 % at the field amplitude of 80 kV/cm up to 10(5) switching cycles. The large and purely electrostrictive effect with the exceptionally good fatigue resistance and thermostability makes this material a great potential to be applied to the environmental-friendly solid-state actuators.