Thermal depolarization temperature, T-d, of ferroelectric oxides Bi0.5Na0.5TiO3 (BNT), where dielectric and piezoelectric signals exhibit remarkable changes, is providing rich research contents but is not well understood yet. Herein, on the domain-scale, we give the direct and clear real-space images of thermal depolarization process on BNT-based complex oxides. As disclosed by the piezoresponse force microscopy (PFM), heating above T-d breaks the poling-induced large-sized-oriented domains into smaller sized polar clusters with different orientations, leading to the thermal depolarization phenomenon. Although the poling-induced domain decays above T-d, the broken domains exhibit a rather larger coherence length than that of the incipient labyrinth-like nano-domains. During the heating process, BNT possesses a structural transition from the long-range-correlated R3c (a(-)a(-)a(-) anti-phase tilting) to the short-range-correlated P4bm (a(0)a(0)c(+) in-phase tilting) phase, which should be the fundamental driving force for the fluctuations of poling-induced large-sized-oriented domains. We expect these results will further promote the understanding about the origin of T-d in BNT-based relaxor ferroelectrics, and provide an intuitive method for the characterization of the thermodynamic and kinetic process in this kind of materials.