Double ReO₃-type fluorides have a great interest in the field of negative thermal expansion (NTE) and luminescent materials. However, their application is limited by the scarcity of quantity, expensive raw materials, and harsh preparation conditions. In this work we have found a new NTE material, CaSnF₆, by applying the concept of the average atomic volume. More importantly, different from the previous solid-phase sintering and direct fluorination methods, the nano CaSnF₆ has been synthesized for the first time by solvothermal method. The results of X-ray diffraction (XRD) and Raman spectroscopy show that a phase transition occurs from rhombohedral ( $R\overline{3}$) to cubic ( $Fm\overline{3}m$) structure at about 200 K, and a strong isotropic NTE (α_v = −15.78 × 10⁻⁶ K⁻¹) appears in the cubic phase. Lattice dynamics calculations from first-principles illustrate that the NTE is due to the transverse vibration of fluorine atoms excited by low-frequency phonons. This work not only broadens the NTE family of fluorides, but also provides a new facile and low-cost fabricati on method for the preparation of NTE fluorides.