The invention provides a welding method for low-alloy and ultrahigh-strength steel with the yield strength being 1100 MPa. The welding method comprises the steps of pre-heating welding base metal with the yield strength higher than 1100 MPa to 150-170 DEG C; selecting the mixed gas of argon gas and carbon dioxide as the protective gas after the welding base metal of the same plate thickness are connected in a butt joint mode, carrying out continuous welding through welding wires with the yield strength higher than 900 MPa and the diameter ranging from 1 mm to 1.2 mm, controlling the interpass temperature of welding beads to range from 150 DEG C to 250 DEG C and controlling the weld heat input to range from 9 KJ/cm to 15 KJ/cm; carrying out hydrogen removal treatment after welding, wherein the preservation temperature ranges from 200 DEG C to 220 DEG C and the temperature preservation time is longer than 1 hour. According to the invention, small heat input and argon-rich mixed gas shielding welding is adopted, the strength of welded joints is guaranteed, and the toughness and the plasticity of the welded joints are improved.