To uncover how polar nanoregions (PNRs) regulate the electrocaloric effect in lead-free ferroelectric thin films, we fabricated Na_0.5Bi_0.5TiO_3-7BaTiO₃ (NBT-7BT) films via metal-organic decomposition. Using variable-temperature piezoresponse force microscopy (PFM), ferroelectric characterization, and three-dimensional phase-field simulations, we systematically probed the electric field- and temperature-driven domain evolution of PNRs. Our results reveal that near the depolarization temperature (T_d≈30 °C), thermally activated PNRs substantially reduce the domain switching energy barrier, efficiently inducing directional switching from in-plane ab domains to out-of-plane c domains. Consequently, both the saturation polarization (P_s) and its temperature sensitivity are substantially enhanced. Under an applied electric field of 500 kV/cm, the film achieves a maximum adiabatic temperature change |ΔT|= 2.0 K in the range of 20-30 °C, as determined by the indirect method based on Maxwell relations. The three-dimensional phase-field simulations accurately reproduce this PNR-induced switching process, showing excellent agreement with experiments and theoretically validating the complete physical chain: “thermally assisted electrical activation of PNRs→domain structure evolution→polarization enhancement→improved electrocaloric performance.” This work elucidates the microscopic origin of the PNR-tailored electrocaloric enhancement and provides robust experimental and theoretical guidance for designing high-performance, lead-free ferroelectrics for solid-state cooling applications.