The butt welding tests of Q345E thin plate with thickness of 3 mm were carried out by using high frequency pulse MAG backing welding mode and arc cold welding mode respectively. The small deformation process and mechanical properties of butt welding joint were studied, the metallo-graphic test was carried out, the causes of welding defects were analyzed, and the laws of welding residual stress and welding deformation were summarized. The results show that high frequency pulse MAG welding is suitable for small deformation butt welding of Q345E steel plate with 3 mm gap. When assembling "0" gap, the process package of backing welding mode is selected; when assembling 1 mm gap, the process package of arc cold welding mode is selected.