The invention discloses a welding deformation control method for a multilayered-structure thin-wall welding case, and a fixture. The method is characterized by welding ten support plates and eighteenwelded joints on each support plate according to a specific sequence through two steps, wherein the welded joints in the first step are welded under the condition of being assembled with the fixture;and the welded joints in the second step are welded without the fixture. According to the welding deformation control method for the multilayered-structure thin-wall welding case, and the fixture provided by the invention, on-site operation can be carried out conveniently, the improvement implementation cost is low, the deformation control of the whole welding process is realized, the welding deformation is effectively improved, and an assembly requirement of a complete machine is met.