Abstract The weld convection and spatter formation in aluminum alloy fiber laser welding are investigated based on numerical simulation and high speed photography results. It is found that a clockwise flow pattern exists in the rear part of the keyhole. The molten metal at the top surface of the rear part flows to the edge of the weld pool, owing to the surface tension gradient. The spatters generate mainly around the keyhole. The spatter formation sequence is that the formation of the swelling around the keyhole caused by the upward flowing molten metal, the increasing of the swelling size caused by the accumulation of vertical momentum, and the spatter ejection from the keyhole when the momentum of the molten metal is sufficient to overcome the surface tension. The recoil pressure caused by evaporation and the shear stress caused by metallic vapor flow are responsible for the spatter formation.