Introduction Ferroelectric ceramics generate a strain property in electric field due to the inverse piezoelectric effect and ferroelectric domain swiching, and these ceramics can be made as key components of actuators. Lead-based ferroelectric ceramics are widely used in actuators, but their high lead level is harmful to human-being health and environment. (K1-xNax)NbO3-based ceramics replace lead-based ceramics for actuators applications. (K1-xNax)NbO3-based ceramics with a multi-phase coexistence structure at room temperature have a high strain due to the possible increasing polarization direction. The classical (1-x)(K1-yNay)(Nb1-zSbz)O3- xBi0.5(Na1-wKw)0.5ZrO3 (0≤x≤0.05, 0.40≤y≤0.68, 0≤z≤0.08, 0≤w≤1) (KNNS-BNKZ) system can be adjusted to have rhombohedral-tetragonal (R-T), rhombohedral-orthogonal-tetragonal (R-O-T), orthogonal-tetragonal (O-T) phases coexisted structures to increase the strain property. When the system is used to construct the coexistence structure, the composition and process need to be finely regulated, especially the raw material calcination or pre-firing process. Pre-firing is a solid-phase reaction of various raw materials at a certain temperature, and the purpose is to synthesize the crystal structure of the target component without raw material components. The activity, particle size and uniformity of the pre-fired powder have a great influence on the difficulty of green sintering and the electrical properties of ceramics. The pre-firing temperature affects the phase coexist structures of 0.96(K0.5Na0.5)(Nb0.96Sb0.04)O3-0.04Bi0.5(Na0.5K0.5)0.5ZrO3 ceramic coming from KNNS-BNKZ system, but the effect of the strain properties of ceramics is not reported yet. In this paper, 0.97(K0.48Na0.52)(Nb0.97Sb0.03)O3-0.03Bi0.5(K0.48Na0.52)0.5ZrO3 (0.97KNNS-0.03BNKZ) with R-T was designed according to KNNS-BNKZ system. KNN-based ceramics with multi-phase coexistence for improving their strain properties were constructed via adjusting the pre-firing temperature. In addition, the effect of pre-firing temperature on the phase structure, dielectric properties, ferroelectric properties and strain properties of 0.97KNNS-0.03BNKZ ferroelectric ceramics was investigated.Methods For the preparation of 0.97KNNS-0.03BNKZ ferroelectric ceramics by a solid-phase sintering method, the ingredients were made according to the chemical formula, and various raw materials were mixed and ground with absolute ethanol in a ball mill with zirconia balls. After 12-h milling, the slurry was dried. The mixture of 0.97KNNS-0.03BNKZ components of 20.0 g in a crucible was calcined at 600-950 ℃ for 3 h. After milling and drying, the powder was pressed into green discs with the diameter of 13 mm. The upper and lower surfaces of the green discs were buried with the same calcination powder, covered with alumina crucible then sealed with alumina powder. Finally, the green discs were sintered in a high-temperature furnace at 1 180 ℃ for 3 h. The ceramic samples were obtained after cooling in the furnace.The samples were not polished and thermally corroded. The surface morphology of the ceramics was determined by a model Smartlab 3 kW scanning electron microscope after direct cleaning and drying. The density of the samples was analyzed by the Archimedes drainage method. After grinding the surface of samples, the phase structure of the ceramics was characterized by X-ray diffracometer (XRD). Their ferroelectric and strain properties were determined at 10 Hz by a model Precision Premier II ferroelectric system. Their permittivity was measured at 10 kHz by a model TZDM permittivity instrument.Results and discussion The effect of calcination temperature (i.e., 600-950 ℃) on the diameter shrinkage, density, relative density, microstructure, crystal structure, dielectric properties, ferroelectric properties and strain properties of 0.97KNNS-0.03BNKZ ceramics was analyzed. The results show that the diameter shrinkage density and relative density of these ceramics firstly increase and then decrease with the increase of the calcination temperature. The ceramics calcinated at 800 ℃ have the maximum relative density (i.e., 94.9%), while the ceramics calcinated at 600 ℃ and 950 ℃ have the minimum relative density (i.e., 80.3% and 80.4%). All of the ceramics have the same distorted orthogonal crystal structure based on the XRD patterns and temperature-dependence of dielectric constant. Their Curie temperature increases from 280 ℃ to 320 ℃, while their dielectric constant increases from 4 520 to 7 769. The maximum polarization strength (Pmax) of the ceramics at 600, 800 ℃ and 950 ℃ is obtained due to the easy domain swiching for the high or small relative density of ceramics. At 950 ℃, the bipolar strain of the ceramics reaches 0.5% at 60 kV/cm because the ferroelectric domain is easy to swiching, and the interacts between V'K\Na-VO defect dipoles and domain swiching. At 600-900 ℃, the strain properties of the ceramics change slightly (i.e., 0.2%-0.3% strain). The unipolar strain properties of ceramics calcinated at different temperatures are similar.Conclusions The pre-firing had little effect on the crystal structure of 0.97KNNS-0.03BNKZ ceramics, but had a great effect on the density and strain performance of the ceramics. When the calcination temperature was 950 ℃, the bipolar strain of the ceramics reached 0.5% at 60 kV/cm, because the ferroelectric domain was easy to swiching and the interacts between V'K\Na-VO defect dipoles and domain swiching.