The invention relates to the field of ship manufacturing tools, in particular relates to an automatic assembling and welding device for a thin-wall T-section, and aims at realizing palletizing management, perceptible material information and pallet transportation of the thin-wall T-section material, realizing the mechanical and automatic production degree in the assembling and welding processes ofthe thin-wall T-section, avoiding manual operation and reducing labor working intensity, realizing automatic centering and positioning of a face plate and a web of the thin-wall T-section, hydraulically assembling and fitting the face plate and the web, removing the original assembling process stage, achieving assembling and welding in the same process, synchronously welding fillet welds at two sides of the face plate and the web of the thin-wall T-section, and well controlling the deformation in high-heat welding condition. With the adoption of the device, the overall construction quality ofthe T-section is high, and the production efficiency is improved.