Bead-on-plate and butt welding of AA6063-T6 aluminum alloy plates with a thickness of 5 mm is carried out by applying a 3500 W Slab CO2 laser. The penetration welding threshold is measured, and the influences of welding parameters on the process stability are studied. Meanwhile, the different joint tensile strength and appearances are tested. The results demonstrate that welds with good appearances could be obtained when welding with the appropriate processing parameters. The as-welded joint tensile strength without filler wire is 180.7 MPa, while the tensile strength is 190 MPa when welding by one side with the filler wire of AlSi12 or AlMg4.5MnZr. However, the joint tensile strength increases up to 200.7 MPa when welding by both sides with the filler wire of AlMg4.5MnZr, which is about 90% of the base-metal tensile strength. The main reasons for the lower tensile strength when welding by one side are that the filler metal can not totally enter the bottom of the weld and there are porosities at the root of weld.