The as-welded 1/8 sector of vacuum vessel (VV) is a necessary component in Chinese Fusion Engineering Testing Reactor (CFETR), and it needs to be within a very tight tolerance. However, it is a challenge to find an effective way to predict and reduce the welding stress and distortion on a full scale prototype of the 1/8 VV due to its large size and complex structure. In this paper, a full size finite element model of the 1/8 VV was built using ABAQUS, and three different tungsten inert gas (TIG) welding sequences were simulated on the 1/8 VV to study the effect of welding sequences on the welding stress and distortion. The results showed that the main welding stress occurred around the welding joints and the inboard segment of 1/8 VV. Meanwhile, the shells near the top and bottom windows distorted perpendicularly to outside of the shells, while the shells around the middle window and the inboard segment distorted perpendicularly to inside of the shells. It was also noted that in three different welding sequences, the maximum distortion all occurred on the shells near the transition structures, the notable difference of the welding stress and distortion was all observed on the shells around windows. Based on the simulation results, an optimized welding sequence was obtained to control the welding stress and distortion in the actual assembly of 1/8 VV.