Abstract: The residual stress and tensile properties of the butt joints of P355NL1 steel for rail vehicles prepared by manual MAG multi-pass welding and robot automatic MAG multi-pass welding before and after heat treatment were tested by X-ray testing system and tensile testing machine.And the effect of stress relief annealing heat treatment on the residual stress distribution and tensile properties of different welded joints was studied.The results show that the longitudinal and transverse residual stress distribution trends of the manual MAG welding and robotic MAG automatic welding joints before and after heat treatment are consistent.They are both approximately symmetrical along the centerline of the weld.There is high residual tensile stress in the weld and heat-affected zone near the weld, and with the increase of the distance from the weld, the residual stress decreases until that the stress in the base material becomes compressive stress. Heat treatment can significantly reduce the peak residual stress around the weld and the area near the weld, while has little effect on the tensile properties of P355NL1 steel welded joints.