The stabilization mechanism of antiferroelectric (AFE) phases in sodium niobate (NaNbO₃, NN) ceramics remains unclear, leading to irreversible AFE–ferroelectric (FE) phase transitions and the need for a high operating electric field (E-field), which significantly limits the utilization of AFE properties. In this work, leveraging insights from density functional theory calculations, we design and fabricate xBi_2/3SnO₃–(1−x)NaNbO₃ ceramics, overcoming these limitations by achieving both a reversible AFE–FE phase transition and a low operating field (< 200 kV/cm). Notably, the optimized composition results in exceptionally low remanent polarization compared with that of conventional AFE systems. Structural analysis reveals a three-stage phase evolution with increasing x: FE Q phase (Pmc2₁, Glazer tilt system: a⁻a⁻b⁺) + AFE P phase (Pbcm, Glazer tilt system: a⁻a⁻b⁺/a⁻a⁻b⁻) (x < 0.02) → pure AFE P phase (x = 0.02) → coexistence of AFE P phase + AFE R phase (0.02 < x < 0.08). The AFE P phase is characterized by a periodically arranged 4-layer multicell structure (~1.65 nm) that is disrupted by 6-layer antiphase boundaries (APBs, ~2.44 nm), which are associated with the dislocation formation and a large energy difference between the P and Q phases (7.20 meV/(f.u.)). These features likely contribute to a reduced domain size and a lower field-induced AFE → FE transition. Furthermore, the stabilization of the AFE R phase is caused primarily by a reduction in the distortion index (from 0.047 to 0.003) and enhanced covalency in A–O and B–O bonds. This study provides new insights and theoretical guidance for the development of low-field-driven reversible phase transitions in AFEs.