The invention relates to a method for welding a carbon steel composite pipe of a thin iron-nickel-based alloy. The method comprises: processing a step on the end part of the composite pipe; adopting the surfacing of argon arc welding on the surface of the step and trimming the surfacing layer; processing a groove; adopting tungsten electrode argon arc welding to carry out three-point positioning of spot welding evenly around a welding seam, and adopting the tungsten electrode argon arc welding to carry out back welding on the root; and adopting manual electric arc welding to carry out filling welding and overlay welding on the upper part of a second layer of back welding. The chemical compositions of a welding material by weight percentage: 0.02 to 0.1 percent of C, 0.2 to 1.0 percent of Si, 1.0 to 2.2 percent of Mn, 20.0 to 31.5 percent of Cr, 40.0 to 60.0 percent of Ni, 8.0 to 14.5 percent of Mo, 0.5 to 2.5 percent of Cu, 0 to 5.0 percent of Co, 0.3 to 4.2 percent of Nb, and the balance being Fe. The welding seam has the characteristics of no sensitivity to offset, favorable corrosion resistance, high strength and favorable plastic toughness.