Keyhole-induced porosity occurring in deep-penetration laser welding is a large issue in laser manufacturing large-thickness components. In order to improve understanding of the keyhole-induced porosity formation and its influencing factors, a three dimensional (3D) transient model is proposed to provide better insight into the correlations of dynamic keyhole behavior with porosity formation under various parameters. Fluid flow, bubble movement and solidification are coupled in this model. Simulation results indicate that the number of bubbles is mainly determined by the frequency of keyhole collapse. The keyhole tends to collapse with increasing laser power and increasing gap size between plates, as well as decreasing travel speed and decreasing beam size. The depth to width ratio of the keyhole and fluid flow also affect the keyhole behavior. However, porosity number not only depends on the bubbles number, but also relies heavily on the evolution efficiency from bubbles to porosity. A good agreement is obtained between simulation and experimental results under the experimental conditions examined here. (C) 2015 Elsevier Ltd. All rights reserved.