The invention discloses a welding-distance height control device and a welding-distance height control method. The device can accurately control the distance between a nozzle of a welding torch and a workpiece, namely the welding-distance height, and belongs to the technical field of welding. The method directly measures the distance between a laser sensor and the workpiece through a laser ranging device. The actual distance is transferred into a PLC control unit after the AD conversion; and after the processing of the PLC, the PLC can automatically select pulse frequency according to the preset speed and the difference between the welding-distance height and the preset height at the moment and output a pulse signal and a rotating direction signal to an alternating motor for servo. The alternating current motor can drive the welding torch to move back and forth, and the distance between the welding torch and the workpiece is adjusted to ensure that the distance is consistent with the preset distance. The proposal is applicable to the piercing welding, has higher control accuracy, a simple structure, and steady effect, and is superior to the prior arc voltage feedback to control the length of an electric arc.