The (1 − x) Na0.5K0.5NbO3–xLiSbO3:0.006Dy3+ were synthesized by a solid state reaction method. The phase structure, microstructure and electric properties were systematically investigated. The orthorhombic–tetragonal (O–T) phase was identified in the composition of x = 0.04 with the help of Raman spectra and photoluminescence response. The ceramic with x = 0.04 mol% LiSbO3 demonstrates optimal properties: Pr = 32.2 μC/cm2, Ec = 12.26 kV/cm, TC = 325 °C, tan δ ≤ 1.5% (250–400 °C). We believe that this work can provide an effective method to investigate the phase transition near polymorphic phase transition (PPT) region. The advantages of simple operation, quick response and pollution-free make it potential to be used in investigating the phase transition near the PPT region of lead-free ceramics.