Abstract: The exploration of highly efficient red-emitting phosphor is crucial for the preparation of warm lighting sources. Based on the low phonon energy and weak electron phonon coupling effect of fluoride, we designed and conducted a novel Mn⁴⁺-activated Cs₂NaInF₆ double-perovskite red-emitting phosphor in this research. The lattice and electronic structures, elemental composition, morphology, band structure, photoluminescence excitation and photoluminescence spectra, quantum efficiency and lifetime were investigated systematically. The results show that the optimal doping amount of Mn⁴⁺ in Cs₂NaInF₆ is 0.27%, which presents broadband excitation and narrowband emissions in blue and red regions, respectively. The corresponding quantum efficiency and lifetime are of 69% and 5.65 ms respectively. Warm WLED device based on the "red-yellow-blue" combination system exhibits a high color rendering index of 93.0 and a low correlated color temperature of 2914 K, evidently indicating the obtained red-emitting phosphor is potentially applicable for warm white light illumination.