The invention discloses a welding process for a steel structural member with a thin bottom plate. The steel structural member is provided with the bottom plate 1 with the thickness ranging from 2 mm to 4 mm, grid-shaped reinforcing ribs 3 are fixed to the lower surface of the bottom plate 1, vertical sides 2 are fixed to the edges of three sides of the bottom plate 1, and one side of the bottom plate 1 is open. The welding process comprises the steps of 1, preparing the bottom plate 1, the reinforcing ribs 3, the vertical sides 2 and process ribs 4; 2, completing installing and welding of the vertical sides on the three sides for forming a U shape and correcting deformation; 3, carrying out spot welding on the bottom plate and the vertical sides and fixedly connecting the bottom plate and the vertical sides for forming an assembly part; 4, carrying out single-face sectional spot welding on the process ribs and the bottom plate; 5, welding and fixing the reinforcing ribs and the vertical sides and welding and fixing the reinforcing ribs and the bottom plate; 6, carrying out secondary welding and fixing on the vertical sides and the bottom plate; 7, carrying out flame straightening; 8, reserving the process rib at the open end, disassembling the other process ribs and then completing all welding work. According to the welding process, deformation generated in the manufacturing and welding process of the steel structural member in the past is controlled, flame straightening is carried out according to the process requirements, and the welding stress is gradually weakened or disappears after flame straightening.