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Multifunctional lead-free double perovskites demonstrate remarkable potential towards applications in various fields. Herein, an environmentally-friendly, low-cost, high-throughput Cs2NaFeCl6 single crystal with exceedingly high thermal stability is designed and grown. It obtains a cubic lattice system in the temperature range of 80–500 K, accompanied by a completely reversible chromatic variation ranging from yellow to black. Importantly, the intriguing thermochromism is proved to own extremely high reproducibility (over 1000 cycles) without a hysteretic effect, originating from its structural flexibility that including (i) the noteworthy distortion/deformation of [NaCl6]5- and [FeCl6]3- octahedra; (ii) order–disorder arrangement transition of [NaCl6]5- and [FeCl6]3- octahedra as the function of temperature. This study paves the way towards a new class of smart windows and camouflage coatings with an unprecedented colour range based on a Cs2NaFeCl6 perovskite.