Abstract: For full welded body ball valve, the last working procedure is welding, and welding distortion must be controlled strictly in order to assure spool rotation and valve seal.Therefore, prior to welding an efficient method must be selected to predict welding distortion accurately to provide preferences for the production planning of full welded body ball valve.Finite element method based on the inherent strain theory was selected to predict welding distortion in multipass girth-butt welded pipes which is similar to valve structure.For multi-layer and multipass welding, the axial distortions of each layer were calculated respectively and the radial distortion was calculated for one time.Through the comparison of welding distortions between calculated and measured, both results are very anastomotic, which proves the accuracy and reliability of the inherent strain method and provides important references for the optimization of welding parameters and the production planning of full welded body ball valve.