Most reported results about phase transition probed by the photoluminescence (PL) spectra of Eu3+ ions are qualitative, but the quantitative analyses of phase transition were presented in this work. We fabricated (Na0.8, K0.2)0.5Bi0.497Eu0.003TiO3 (NKBT20:Eu) ferroelectric ceramics and investigated the phase structures and the PL spectra of NKBT20:Eu ceramics before and after poling at different electric fields. The PL spectra indicate a phase transition from tetragonal phase (T phase) to rhombohedral phase (R phase) within NKBT20:Eu ceramics after poling, consistent with the analyses by X‐ray diffraction (XRD). Moreover, the emission intensity ratios of the “hypersensitive” transition to the magnetic dipole transition could be used to calculate the fractions of the phase transition quantitatively, which were further confirmed by XRD Rietveld refinements. This study provides a different perspective to investigate the phase transition induced by electric field for NKBT20:Eu ceramics, qualitatively and quantitatively, and the method is expected to be useful in more cases of phase analyses.