In a method and apparatus for making welded large pipes, a leading end of a hot strip is connected to a trailing end of a leader strip and then subjected to a two-stage leveling for strip flatness in transverse direction and strip flatness in longitudinal direction. The entire surface of the hot strip including strip edges thereof is inspected by ultrasound and the strip edges are prepared in four stages before being pre-bent. The hot strip is then shaped into a slotted tube and the strip edges are welded along the inner and outer sides by laser to produce the pipe. Online process diagnostic is provided for monitoring the welding step and the finished welded pipe diameter is measured through online measurement. Online determination of a profile of a welded seam configuration and computer tomographic determination and evaluation of flaws inside the pipe, as well as ultrasonic inspection of the welded seam are further provided.