The invention relates to the technical field of bridge detection, discloses a crack detection device for a double-layer supporting bridge, and solves problems that in the prior art, time and labor are wasted due to the fact that crack detection is carried out mainly through a handheld detector, personnel need to go below the bridge when crack detection is carried out on the bottom face of the bridge, personal safety of the detection personnel is threatened, efficiency is low, and practical use is not convenient. The bridge comprises a bridge body and a moving seat, wherein the moving seat is located above the bridge body, a lifting seat is arranged on one side of the moving seat, a rectangular hole is formed in one side of the lifting seat, a movable plate is arranged on one side of the lifting seat, and the movable plate penetrates through the rectangular hole; the movable plate and the lifting seat are connected through a moving-out movable mechanism; when a crack detector body rotates, the crack detector body slides relative to the rotating plate, different positions of the bottom of the bridge body can be detected in all directions, operation steps are simple and convenient, personnel do not need to go below the bridge for detection, and safety is improved.