The non-stoichiometric Li₃Mg₂Sb_1-xO₆ (0.05 ≤ x ≤ 0.125) compounds have been prepared via the mixed oxide method. The influences of Sb nonstoichiometry on the sintering behavior, microstructure, phase composition along with microwave dielectric performances for Li₃Mg₂Sb_1-xO₆ ceramics were studied. Combined with X-ray diffraction (XRD) and Raman spectra, it was confirmed that phase composition could not be affected by the Sb nonstoichiometry and almost pure phase Li₃Mg₂SbO₆ was formed in all compositions. Appropriate Sb-deficiency in Li₃Mg₂SbO₆ not only lowered its sintering temperature but also remarkably improved its Q×f value. In particular, non-stoichiometric Li₃Mg₂Sb_0.9O₆ ceramics sintered at 1250 ℃/5 h owned seldom low dielectric constant ε_r = 10.8, near-zero resonant frequency temperature coefficient τ_f = -8.0 ppm/℃, and high quality factor Q×f = 86,300 GHz (at 10.4 GHz). This study provides an alternative approach to ameliorate its dielectric performances for Li₃Mg₂SbO₆-based compounds through defect-engineering.