A three-dimensional (3D) transient heat transfer and fluid flow model for laser-MIG hybrid welding is developed to investigate keyhole dynamics and temperature and fluid flow fields in weld pools. The effect of the laser-arc tandem relative position on the heat transfer and fluid flow of the weld pool is elucidated. The model considers the effect of the welding torch angle on droplet transfer and the effects of multiple reflections on laser energy distribution. The results show that the downward flow along the keyhole wall forms backward flow and counterclockwise circulation after reflection on the bottom of the weld pool. The backward flow transports heat and momentum to the rear portion and increases weld pool volume. The counterclockwise circulation impinges on the keyhole back wall and reduces the keyhole stability. In laser leading configurations, the droplet and arc pressure impact behind the keyhole and cause two flow patterns, namely the forward and outside flows. The forward flow enhances the impingement of counterclockwise circulation on the keyhole back wall, and the collapse of the keyhole becomes more severe. The outside flow transfers heat to both sides of the weld pool and leads to a wider weld.