Abstract: High power laser welding provides an effective method for high-quality and high-efficiency welding of medium-thick aluminum alloy plates, but the porosity defects are prominent. In this study, a research on the process control of the porosity defects during high power oscillating laser welding of medium-thick aluminum alloy plate is carried out. Particularly, the influence of oscillating amplitude and frequency on the porosity of joints is clarified. Moreover, a welding parameter optimizing method is proposed. The results show that the oscillating amplitude mainly affects the lateral distribution of the beam movement trajectory and the aperture area of the keyhole, while the oscillating frequency mainly affects the laser beam movement speed and the number of oscillating times to the rear molten pool. The porositiy decreases as the oscillating amplitude and frequency increases, the weld penetration, however, is difficult to meet the requirements. The correlation of the process parameters, laser power, oscillating frequency, oscillating amplitude and welding speed can be established by controlling the beam movement trajectory and energy density, and the best process interval with large penetration and less porosity can be obtained. Finally, a weld with less porosity was obtained, and the penetration depth reaches 6.4 mm.