The invention relates to a welding method used for CO2 gas shielded welding of a large-specification ultrahigh-strength steel plate. According to the technical scheme adopted by the invention, the welding method comprises the following steps: processing a groove in an unloaded to-be-welded large-specification ultrahigh-strength steel plate, wherein the groove is in the shape of double Us with truncated edges; welding by adopting a CO2 gas shielded welding method; and carrying out nondestructive examination on a welding seam by adopting ultrasonic waves, and carrying out mechanical property detection on a welded joint. The welding method adopts welding wires of two strength grades, wherein CO2 gas shielded welding wires of 420MPa-600MPa grade are arranged at odd layers, welding wires having strength equal to that of a parent material are arranged at even layers, and the welding wires are sequentially welded layer by layer. The to-be-welded large-specification ultrahigh-strength steel plate is a steel plate with a thickness being 50-100 mm and yield strength being 690-980MPa. The welding method has the characteristics of a simple process, small labor intensity, low welding cost and high welding efficiency; and a welded joint formed by the welding method has good comprehensive mechanical properties, and improves impact toughness of welding seam metal.