Abstract: In order to reduce the welding residual stress, researchers adopted tempering weld-pass treatment for the improvement of the class III vessels in an expansion project.Due to the optimization and evaluation of welding process, the researchers conducted a comprehensive test and evaluation by X-ray method on six Q345 steel welding test plates(600 mm×300 mm×30 mm), with three kinds of processes(welding and two kinds of new tempering welding treatment) and 2 patterns(flat butt and T-joint welded plate) and considering the weld metal polished(side B) while the other side(side A) not.The results showed that the welding residual stress in 1 and 3 T-plate decreased by 46% and 36% respectively than which is not with tempering welding treatment.Tempering weld-pass treatment reduced the longitudinal stress(LS), and the transverse stress(TS) became the first principal stress.The residual stress of A-side was 25% higher than that of B-side.To reduce the residual stress, the weld metal can be removed in practical engineering applications.