In this work, 0.8Na(0.5)Bi(0.5)TiO(3)-0.2K(0.5)Bi(0.5)TiO(3) (BNKT) is studied by electrical characterizations, insitu Raman spectra in conjunction with the phenomenological Landau predictions to reveal the local phenomena. An electric field-induced nonergodic relaxor-to-ferroelectric phase transition and domain switching start to take place at a critical field of 3 kV/mm. The polar phase was disrupted into the disordered relaxor phase at ferroelectric-relaxor transition temperature (T-FR) along with the discontinuous phonon anomalies caused by the decoupling between the off-centered Bi and Ti cations. The R3c and P4bm polar nanoregions (PNRs) can coexist in a wide temperature range and can be transformed reversibly below the temperature of dielectric maximum (T-m). The broad Raman bands were found to persist in high-temperature paraelectric state induced by the existence of off-centered Ti cation and ferroically deformed TiO6 octahedra. A phenomenological Landau model and local structural evolution sequence at different thermal history were proposed to further elucidate the underlying mechanisms. This work will be conducive to understanding the local structure-physical property correlations of high-performance lead-free piezoelectrics.