The invention discloses a single-cantilever-crane gantry crane which comprises a gantry, wherein a walking mechanism is arranged at the lower end of the gantry, and a turntable is arranged on the gantry; the turntable is respectively provided with a propeller strut, a machine room, a cab and a cantilever crane; the machine room is internally and respectively provided with a lifting mechanism and a jib lubbing mechanism; the lifting mechanism and the jib lubbing mechanism are respectively provided with a lifting cam limit and a jib lubbing cam limit; a contact signal which can induce the maximum cam limit positioning amplitude exceeding the lifting cam limit in the lowering process of the cantilever crane is increased on the lifting cam limit; and a cantilever crane angle sensor, the lifting cam limit and the jib lubbing cam limit are connected with an electric control system respectively through leads. The invention has the advantages that in the process of putting the cantilever crane on the ground, the contact signal can be increased to prevent a hook pulley block from colliding with a pulley block arranged at the other end of the cantilever crane, thereby the service life of the hook pulley block is prolonged.