The invention provides a welding device for compounding an ultrasonic focusing sound field with molten pole arc welding for improving the energy density of ultrasonic, changing the arc shape and the energy density and realizing high-efficiency high-quality welding. The welding device comprises a molten pole welding wire, a conductive pole, an ultrasonic transducer, an axle center through hole ultrasonic amplitude transformer, a welding nozzle, a focusing surface, a welding arc, a workpiece to be welded, a welding power supply and an ultrasonic power supply, wherein the molten pole welding wire passes through the conductive pole and is in sliding contact with the conductive pole, the ultrasonic power supply is connected with the ultrasonic transducer through a cable, the ultrasonic transducer is connected with the ultrasonic amplitude transformer through a bolt, and the focusing surface is positioned at the lower end part of the ultrasonic amplitude transformer. The invention ensures that the arc is compressed to improve the arc energy density, increase weld penetration and improve weld formation. The invention can change the arc shape and the heat generation characteristic and the transition form of molten metal, realizes the high-efficiency high-quality welding, and enlarges the application range of the arc welding.