The invention discloses a construction engineering wall surface cavity detector. The detector structurally comprises a hammer head, a connection head, a magnetic head, an elastic structure, a telescopic rod, a pressing structure and a grip rod; one end of the telescopic rod is fixed with the grip rod and the other end of the telescopic rod is vertically welded together with the middle of the connection head; the hammer head and the magnetic head are respectively mounted at two horizontal ends of the connection head; the elastic structure and the pressing structure are respectively mounted at the joints of the telescopic rod and the grip rod; the pressing structure and the telescopic rod cooperate with each other; and the other end of the elastic structure is connected with the connection head. According to the detector, the elastic structure, the telescopic rod, the pressing structure and the grip rod are jointly matched, and a fiber rod is bent and rebounded to urge the hammer head tocarry out tapping and detection, so that the force is more concentrated and stable and points can be accurately aligned; and before and after height adjustment is carried out by the telescopic rod, apressure pointer is used to apply pressure to the telescopic rod to prevent the telescopic rod from relative sliding , so that the accuracy of the tapping is improved.