The invention discloses a submerged arc welding method for a K-shaped groove of an 80mm extra-thick steel plate with an excellent plate shape after welding. The method comprises the following steps of: taking a K-shaped symmetrical groove machined by machinery as the groove type, wherein a truncated edge is not remained at the root, and the assembling clearance of the groove is 2-3mm; performing backing welding on a front root weld pass of a K-shaped groove test plate by adopting a solid gas shield welding wire during backing welding, so that partial weld metal penetrates through the reverse side of the groove after the root weld pass is penetrated, the backing welding is finished, and employing mixed gas protection of argon and carbon dioxide during welding; performing positive submerged arc welding after backing welding is finished, turning over and performing back chipping while welding to 2 / 3 depth of the groove, performing reverse submerged arc welding until the reverse welding line is totally filled, and finishing welding of the residual 1 / 3 depth on the positive side after the welding on the reverse side is completely finished. The method is easy and convenient to operate, the turn-over frequency is reduced, the labor intensity is reduced, the working efficiency is improved, the residual deformation of a soldering block is small, and the welding quality is high.