A group welding method for five plates on the roof of a CRH5 motor train unit is characterized by comprising the following steps: step one, a forward-assembly reversible deformation supporting block is pre-placed at the position of a middle top plate of the roof, wherein the height of the forward-assembly reversible deformation supporting block is 5 to 7mm; step two, the welding sequence is that first performing backward-assembly group welding and then performing forward-assembly group welding; when performing the backward-assembly group welding, two welding lines in the middle are welded first, and then two welding lines at two sides are welded; when performing the forward-assembly group welding, the two welding lines at the two sides are welded first and then the two welding lines in the middle are welded. According to the group welding method provided by the invention, the whole assembly and the whole welding of the five plates on the roof are realized, so that all butt welding lines are welded in the same working procedure, and the size is qualified and adjusting and repairing is avoided. The separation of the butt welding lines and lap welding lines of roof compounding is realized, the group welding time of subsequent whole roof compounding is reduced to 360 minutes from 960 minutes, the whole roof adjusting and repairing time is reduced to 45 minutes from 180 minutes, and the operation of staff is reduced to 2-riveting and 1-welding from 4-riveting and 2-welding, the assembly difficulties in the subsequent roof compounding, the labor intensity and the welding deformation are reduced, and the labor efficiency and product quality are improved.