In this work, the 0.99Ri(0.505)(Na0.8K0.2)(0.5-x)TiO3-0.01SrTiO(3) (BNKST(0.5-x)) nonstoichiometric ceramics with x = 0.015, 0.02 and 0.025 were synthesized by the normal solid-state sintering method. The structure, ferroelectric and electrostrictive properties of (Na, K) nonstoichiometric ceramics were investigated. The electrostrictive coefficients (Q(33)) increase from 0.0192 m(4)/C-2 for x = 0 stoichiometric sample to 0.027, 0.0255 and 0.0248 m(4)/C-2 for x = 0.015, 0.02 and 0.025 nonstoichiometric samples, respectively, which are comparable with that of lead-based electrostrictors. The disordered local structure and the interactions between the highly dynamic PNRs play important roles on the enhanced electrostrictive effects of nonstoichiometric samples. Furthermore, the Q(33) values of all the samples show temperature-insensitivity up to 120 degrees C, rendering them suitable for actuators and high-precision positioning devices. Our work may provide a new approach to quest for high-performance NBT-based lead-free electrostrictive materials by means of A-site compositional modification.