The lead-free ceramics ($1?x$)(0.94${\text{Bi}}_{0.47}$${\text{Na}}_{0.47}$${\text{Ba}}_{0.06}$TiO₃-0.06BiAlO₃)-$x$AgNbO₃ (denoted as BNBTA-$x$AN) were synthesized via a solid-state sintering method. The effect of AgNbO₃ doping amount on dielectric properties of the ceramics was studied systematically. X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscope were used to detect the structure of the ceramics. Temperature-dependent dielectric spectra, frequency-dependent dielectric constant and alternating current (ac) electric conductance at various temperatures were measured. The doping of AgNbO₃ greatly reduces dielectric constant around Curie temperature and thus enhances the temperature stability of the dielectric constant. The ceramic BNBTA-0.03AN exhibits excellent temperature-stable dielectric properties with temperature coefficient of capacitance (TCC) $\le \pm$15% between 55^°C and 418^°C with temperature window 363^°C and small changes of dielectric constant and dielectric loss from 100 Hz to 1 MHz at different temperatures. The obtained ceramics are expected to be used in high-temperature capacitors due to its excellent temperature stability.