Development of highly thermally stable broadband near-infrared (NIR) luminescence materials is crucial for advancing the prolonged stable application of smart NIR light sources. In this study, a zero-thermal-quenching and reversible temperature-dependent broadband NIR-emitting Cs2NaAl3F12:Cr3+ phosphor is demonstrated, benefiting from its stable polyhedron-cluster-building rigid structure. The excellent thermal stability of Cs2NaAl3F12:Cr3+ is rooted in its stable [Al6Na4F45] cluster building unit, which provides a rigid structure with a weak electron-phonon coupling effect and a wide band gap with a huge thermal activated barrier. Such characteristics are well revealed by multiple studies on crystal structure, electronic structure, Huang-Rhys factor S, configuration coordinate model, and Debye temperature. The incorporation of Li or K instead of Na weakens the luminescence thermal stability, directly proving the importance of the stable [Al6Na4F45] cluster for stable Cr3+ substitution and rigid structure construction. Furthermore, Cs2NaAl3F12:Cr3+ presents much superior thermal stability compared to traditional rigid garnet-type fluorides Na3X2Li3F12:Cr3+ (X = Al, Ga, In). A high-power NIR LED is presented, utilizing the high quantum efficiency (∼71%) and extremely thermally stable broadband NIR emission around 750 nm of Cs2NaAl3F12:Cr3+. It realizes clear vein and cartilage imaging in the human hand, demonstrating its potential in medical diagnosis applications. This result provides important insights for designing new-type rigid crystal structures using stable polyhedron clusters as basic units, advancing the development of highly thermally stable NIR-emitting phosphors.