In order to accurately simulate the MIG welding process of medium thick plate under complex shape molten pool, a combined heat source model with special spatial distribution of arc + droplet was established by extracting the weld fusion line and fitting the polynomial equation. Based on the model and the double ellipsoid heat source model, the finite element simulation of 6 mm thick 6082-T6 aluminum alloy MIG welding was carried out, and the welding temperature field and post-welding stress field were calculated. Combined with the simulation results of the two heat source models, a comparative analysis was made from the actual weld shape,thermal cycle curve and stress distribution measurement values.The results show that the combined heat source model with weld fusion line has fewer regulating parameters, the temperature field can better reflect the complex shape of the weld pool, and the stress field is more consistent with the measured value, so it is suitable for the simulation of MIG welding process of medium thick plate with groove opening.