The invention discloses an assembly welding method for an ultrathin saw blade with the large diameter, which comprises the processing steps of forming, preheating, welding, thermal insulation and slow cooling, etc. The invention relates to an assembly welding method of a saw, the selection of welding materials, an assembly welding structure of the saw, a way of preheating and slow cooling as well as the design content of parameters, welding process, and the technique. The diameter of carbon tool steel, alloy tool steel, and spring steel which are used in welding is between 2,800mm to 5,200mm, and the materials are used for producing the ultrathin saw blade with large diameter and the thickness of 11mm to 18mm. The invention has the following advantages of simple device, popular welding method and easy mastering for the operation process; the design of the assembly welding structure and the welding process of the saw is reasonable, the welding can be carried out without the need of spot welding and fixing for a welding seam, the welding stress is distributed uniformly, after welding, the whole saw blade is neat without deformation and the welding seam has no welding cracking; the reasonable selection of the welding materials meets the performance requirements that the welding seam can bear, and keeps the same color of the welding seam and substrate.