The invention relates to an online ultrasonic welding device for a waistline of a pull-up diaper and a welding method. With the adoption of the welding device and the welding method, the welding intensity of the welded waistline can be completely changed on the premise that the welding length of the waistline of the pull-up diaper remains, and the welding pressing lines of the waistline of the pull-up diaper are consistent. The online ultrasonic welding device comprises a frame assembly, wherein two bottom roller assemblies are correspondingly positioned on the frame surfaces of the frame assembly and separated through a vertical plate in the frame assembly; a supporting plate assembly is positioned on the two bottom roller assemblies; horizontal belt conveying assemblies are positioned below the supporting plate assembly; two or a plurality of ultrasonic welding devices are mounted at two sides of the vertical plate in the frame assembly; two pressing conveying assemblies are correspondingly positioned at two sides of the horizontal belt conveying assemblies; two transition wheel pressing wheel assemblies are positioned above the horizontal belt conveying assemblies at two sides of the vertical plate in the frame assembly. The online ultrasonic welding device for the waistline of the pull-up diaper has the advantages that 1, the structure is simple, the manufacturing cost is low, and the mechanical assembling difficulty is small; 2, the tension of materials is easily controlled in the welding process; 3, the welding effect is conveniently observed, and the welding intensity is high; 4, the operation is stable; and 5, the maintenance is simple and convenient.