Flux-copper backing process welding is a high-speed, one-sided automatic welding process that is a type of submerged arc welding. Although it has been widely applied for the joining of flat skin plates in shipbuilding, a couple of problems with this process still exist: longitudinal cracks occur in the vicinity of the end point, and transverse cracks occur on the back surface. The data in this study were collected in the production site of Ishikawajima-Harima Heavy Industries Company Ltd, Kure Shipyard. The causes of failures were examined, and countermeasures were studied. Concerning the longitudinal cracks, the behavior of plate deformation during welding was investigated, and the end tab plates and the tack welding method were improved in order to reduce the rapidly increased transverse distortion of the joining plates. Concerning the transverse cracks, the dispersal figure of the reverse flux to form the backing bead was observed, the relation between the flux spraying height and the welding quality was investigated, and the height was standardized. In addition, welding heat input was also examined and regulated to avoid the transverse cracks.