A three-dimensional (3-d) computational fluid dynamics (CFD) based model in which a ray-tracing algorithm was combined with the volume of fluid (VOF) method was used to simulate a pulsed Nd: YAG laser /tungsten inert gas (TIG) hybrid welding process. The calculated weld cross sections in pure laser welding and hybrid welding process were validated by the experimental results, respectively. Then, the variations of keyhole shape and laser energy coupling behaviour in the laser-arc hybrid welding process were calculated at three different laser-arc distances. It was found that the laser-arc distance would affected the shape of keyhole generated in the laser-arc hybrid welding process, thus influencing the laser energy coupling efficiency. In the initial stage of each laser pulse, small laser-arc distances favoured the generation of keyholes and multiple reflections of laser beam within keyhole. However, as the laser irradiation time increased, the smaller the laser-arc distance, the more liquid metal were found around the keyholes, which was more favourable to the expansion of keyhole in radial direction. As a result, the aspect ratio of keyhole, the reflection time of laser beams in keyhole, and the laser energy absorbed by keyhole wall decreased. Given that the interaction between laser and arc was neglected, the maximum keyhole depth increased with reducing laser-arc distances, while the total laser energy coupling efficiency decreased as the laser-arc distance was reduced.