Abstract: Q345 low alloy steel is the main manufacturing material of stressed structural part of large-scale lifting machinery. The residual stress of the base material and weld seam is the important factor that affects the fatigue life of the crane structure. The residual stress of Q345 steel butt weld seam was tested and analyzed by X-ray diffraction method, and then residual distribution curve of Q345 steel surface was obtained. The results show that the gradient of residual stress on the weld-fusion line was considerable, and emphasis should be placed on this area.