The utility model discloses a crack width automatic monitoring device. The device comprises a rack, an elastic body, a movable knife edge seat, a horizontal lever and a fixing assembly, wherein a support point fixing knife edge is arranged on the bottom part of a vertical beam of the rack, both an upper surface and a lower surface of a rectangular beam of the elastic body are bonded with two sensing elements, one end of the elastic body is connected with the horizontal lever while the other end is connected with the movable knife edge seat, the top part of the movable knife edge seat is connected with the bottom part of the rack, one end of the movable knife edge seat is provided with a knife edge groove while the other end is connected with the elastic body, one end of the horizontal lever is connected with the elastic body while the top part of the other end is provided with an abutting part, a support point moving knife edge is arranged on the bottom part of the horizontal lever, and the fixing assembly is used for fixing the support point fixing knife edge of the rack and the support point moving knife edge of the horizontal lever onto two sides of a crack to be tested. With adoption of the above device, the width of cracks in a concrete structure can be precisely, automatically and chronically monitored under various weather and environment conditions.