Abstract: 20 mm thick Q345E steel plate was welded by oscillating laser-MAG hybrid welding process, and the conventional laser-MAG hybrid welding was also applied for comparison. The results show that the oxidation of the bottom bead can be effectively avoided and the defect of the side wall insufficient fusion in the groove can be eliminated when the oscillation frequency of 200 Hz and amplitude of 1mm are added to the laser beam. The results of X-ray inspection demonstrate that the porosity is effectively suppressed by oscillating beam. In addition, the laser beam oscillation can accelerate the flow of molten pool and break the dendrite, which can promote the heterogeneous nucleation in the crystallization process, so as to refine the microstructure.