The CFETR equatorial port extension is a double-walled stepped structure like the main body of the vacuum vessel (VV). During the 1/8 VV mock-up fabrication, this part will be connected to the main vessel by welding. The high-density welded joints in the equatorial port extension mock-up will badly affect the overall profile of the whole structure. The shrinkage of the open end makes the fit-up accuracy unable to meet the requirement. In this paper, to accurately predict the welding distortion of the equatorial port extension mock-up, the distortions of local models were evaluated by the thermo-elastic-plastic (TEP) method first to validate the results obtained by the inherent strain method. Then the welding distortions in three stages without external constraints conditions were simulated and analyzed for the design of the welding tooling. The comparison between the welding deformation with and without tooling presents that the dimensional accuracy of the whole structure is basically within the tolerance, proving that the designed tooling is effective. The simulated results were also validated by the actual measured results. The predictions and designs in this article can provide theoretical guidance for the practical processing of the CFETR equatorial port extension mock-up.