Abstract: On the basic of actual requirement to improve design factor(DF) from 0.72 to 0.80 in engineering,the stress distributions in girth welded joint with concavity imperfection of X80 gas pipeline were studied under the conditions of the hydrostatic testing and the normal operating of pipeline,respectively, and the influences of the increase of DF on the stress concentration of concavity imperfection were analyzed quantitatively.It is found that the axial stress peak at concavity imperfection for 0.80 DF can be lowered to a level equivalent to that for 0.72 DF by controlling the depth of concavity imperfection,and the girth stress concentration can be also improved.Hence,some research evidences and references are provided for the increase of DF of gas pipeline based on the view of controlling the welding quality for girth welded joint.