Among the lead-free piezoceramics, ($1-x$)BiFeO${}_3-x$BaTiO₃ (BF-BT) is considered a promising candidate for high-temperature piezoelectric materials owing to its high Curie temperature ($T_C>400^{\circ }$C) and good electromechanical properties. In this work, the hydrothermal synthesis method was used to prepare the precursor powders of BiFeO₃ and BaTiO₃, and then the mixed powder compacts with the chemical composition of 0.7BF–0.3BT were sintered under pressureless conditions. The influence of the hydrothermal reaction times (12–24$$h) of BiFeO₃ on the structures and electric properties of the sintered ceramics was instigated. First, all the samples synthesized with the tetragonal BaTiO₃ and BiFeO₃ powders were identified with relatively stable dielectric properties. As the hydrothermal reaction time to synthesize BiFeO₃ increased, the dielectric properties as well as the temperature stability of the 0.7BiFeO₃–0.3BaTiO₃ ceramics also improved. At the condition of a hydrothermal reaction time of 24$$h, the sample obtained possesses both the lowest temperature coefficient of dielectric constant ($T{k}_{\epsilon }=1.53\times 10^{-2}/{}^{\circ }$C between RT and $300^{\circ }$C) and the highest Curie temperature ($T_C=471^{\circ }$C at 100$$kHz). Moreover, at high temperatures, it exhibits a higher AC impedance than others. The calculating result based on the resistive constant-phase-element model (R-CPE) circuit model showed that the grain boundary of the 0.7BF–0.3BT ceramics contributes more resistance to the conductivity at high temperatures. In summary, the hydrothermal reaction proved to be a useful way that achieves the preparation of single-phase 0.7BF–0.3BT ceramics with improved electrical properties.