Driven　by　the　breakthrough　development　of　lead　(Pb)-based　halide　perovskites,　halide　double　perovskites　have　also　emerged　as　remarkably　multipurpose　materials　owing　to　their　environmental　friendliness,　abundant　physicochemical　properties,　and　structural　tunability.　However,　despite　their　recent　soaring　progress,　the　realization　of　switchable　dielectrics　in　this　fascinating　family　remains　considerably　less　explored.　In　this　study,　centimeter-sized　single　crystals　of　a　single-layer　halide　double　perovskite　(3-bromopropylaminium)(4)AgBiBr8　(1)　with　dimensions　up　to　20　mm　x　17　mm　x　5　mm　have　been　successfully　synthesized　by　using　the　solution　cooling　method.　The　single　crystal　undergoes　a　ferroelastic　phase　transition　of　mmmF2/m　defined　by　the　Aizu　notation,　which　is　attributed　to　the　synergistic　interaction　between　octahedral　distortion　and　flexible　molecular　motion　according　to　structural　analysis.　Noteworthily,　it　possesses　desirable　ferroelastic　phase　transition-triggered　switchable　dielectric　properties　near　the　high　temperature　of　373　K,　which　can　be　switched　between　two　stable　dielectric　states.　Additionally,　1　possesses　an　indirect-bandgap　semiconducting　feature　and　exhibits　an　X-ray　response　with　a　sensitivity　of　up　to　517　mu　C　Gy(air)(-1)　cm(-2)　at　a　40　V　bias　voltage.　We　believe　that　this　study　provides　another　efficient　application　path　for　Pb-free　halide　double　perovskites　with　intelligent　functions.