A new cyanometallate-based double perovskite-type compound has been synthesized with the formula of (HTz)₂[KCo(CN)₆] (HTz = thiazolium cation, 1). The basic building units of the framework are Co(CN)₆ and K ions. One K ion coordinates with six adjacent cyanide N atoms to form regular octahedral geometry. Meanwhile, the HTz rings exist in the cage. It undergoes two-step reversible structural phase transitions at 237 K and 210 K along with switchable dielectric transformation. Variable-temperature single-crystal structural analysis reveals that the relatively small displacements and/or thermal vibrations of the HTz rings cause the phase transitions. Meanwhile, the switchable dielectric constant of 1 means a dielectric transition between a low-dielectric state (switch off) and a high-dielectric state (switch on). This characteristic may enrich the family of stimuli-responsive materials and help find potential applications of switchable dielectric materials.