The invention relates to a building expansion and contraction device, and provides an expansion and contraction device with a lower supporting platform. The expansion and contraction device comprises two edge beams located on the two sides of an expansion joint and arranged oppositely, and further comprises a waterproof rubber strip clamped between the two edge beams. Two sliding plates in one-to-one correspondence with the two edge beams are arranged under the waterproof rubber strip, each sliding plate extends horizontally to the other edge beam from the corresponding edge beam, and the ends, away from the corresponding edge beams, of the two sliding plates are overlapped. According to the expansion and contraction device, the two sliding plates provide vertical supporting for the waterproof rubber strip in the vertical direction, and thus the situation that after gravel enters the expansion joint, wheels roll the gravel when passing, the gravel extrudes the waterproof rubber strip, and consequently the waterproof rubber strip sheds from the edge beams can be avoided; besides, the sliding plates can generate vertical resilient acting force, and thus wheels can bring out gravel after passing.