With the addition of transparent glass, the keyhole behavior is directly observed in laser welding of aluminum alloy. Based on the Fresnel absorption model, multiple reflections of the laser beam and the adiabatic bubble model, a three-dimensional model is built to research the keyhole behavior and the keyhole-induced porosity formation. The numerical and experimental results show that the process of keyhole formation and keyhole collapse are similar. Downward flow is observed in the front of the keyhole wall, and a clockwise vortex is found in the rear part of the weld pool. The formation of bulges at the rear keyhole wall, molten metal flows toward the front keyhole wall and the liquid bridge formed by the connection between the rear keyhole wall and front keyhole wall are the main effective compositions for the formation of keyhole collapse. The keyholeinduced porosity formation mechanism can be sketched that the bubble is formed due to the keyhole collapse and then captured by solidication interface.