This paper reports a mechanism understanding of how the sinusoidal beam oscillation reduces the porosity defects during laser welding of 7075 aluminum alloy. In this study, the coupled time dependent keyhole and weld pool dynamics as well as the porosity formation during the oscillating laser welding process were simulated. Also, the keyhole and weld pool behaviors were experimentally investigated using high speed imaging and the porosity rates of the welds with or without beam oscillations were characterized. The results indicated that the beam oscillation modulated the fluid flow in the weld pool into a swinging mode and stabilized it. In this mode, the average flow velocity exhibited periodical oscillation with the double frequency of that of the laser beam oscillation. The velocity oscillating amplitude decreased as the frequency of beam oscillation increased. When the oscillating frequency of the fluid flow increased to 100 Hz, the swinging flow with a small oscillation amplitude could be obtained. The results also indicated that with the beam oscillation the keyhole stability was improved and the porosities were reduced in the welds. This is mainly because the keyhole diameter was significantly increased and the surface tension effect, the major cause responsible for the keyhole oscillations, was weakened. However, in cases with beam oscillation, there were still some porosities in the welds. An optimization method to reduce the porosities by limiting the time of laser staying in unstable area was proposed. Porosity free welds were successfully made. This study could help improving the understanding of the weld pool dynamics in laser welding and guide the selection of the process parameters. (C) 2019 Elsevier Ltd. All rights reserved.