In view of the complex and difficult to control the instability deformation caused by the Q235 thin plate TIG welding, this paper studies the judgment method of the thin plate instability, clarifies the instability deformation state of the thin plate in the whole welding and cooling process, and analyzes the law of instability evolution of thin plate welding.Research shows that during the TIG surfacing and cooling process of the Q235 thin plate, the instability deformation state gradually changes from the arch to the reverse saddle shape.The instability deformation is generated by the compressive stress around the weld and occurs during the welding heating process. The displacement-type internal stress loading method the of the thin plate makes the instability deformation process exist between the two deformation inflection points.