Abstract: Because 400 m³ light ends spherical tank is a shell structure with weak rigidity, the angular deformation of weld seam of the tank is easy to occur as its curvature reduced after frequent handling. The field inspection results show that the most angular deformations (arch deformation)of weld seam are between 0~5 mm, the individual location is 6. 5 mm. The countermeasures to be taken against the deformation the paper given includes: depending on the external forec of wedges on clamp device; installing anti-deformation arc-plate and welding according to proper welding procedure and welding parameters.