Due to the existing of weld pore defects in laser welding of medium-thick aluminum alloy, the properties of welded joint are difficult to meet the mechanical properties and application requirements. In this paper, the weld pore defects in oscillating laser welding of medium-thick aluminum alloy are analyzed by the numerical method. To reveal the suppression mechanism of weld pore defects, a dynamic model of molten pool and keyhole in the oscillating laser welding is established and the influence of the oscillating laser beam on the dynamic behaviors of molten pool and keyhole during the weld formation process is discussed in details. The results demonstrate that the opening area above the keyhole is increased and the direction of fluid dynamic pressure near the keyhole is changed under the effect of oscillating laser beam, which improves the keyhole stability. The oscillating behavior of keyhole results in a smaller clockwise flow region in the lower part of the molten pool, which makes the movement range for bubbles smaller and hence increases the probability of bubble escape. The suppression mechanism of weld pore defects in the oscillating laser welding of medium-thick aluminum alloy is revealed for guiding significance for improving the properties of welded joint.