Abstract: In aircraft manufacturing industry, the aluminum alloy is mainly used in the skin, beam, purlin and frame structures. Laser welding of 1.2 mm thick LY12 aluminum alloy sheets was conducted with multimode fibre laser. The influence of laser welding parameters on aluminum alloy weld shape was studied, and the fracture appearance and microhardness of welded joint were analyzed. The results show that the weld appearance with large power and high speed continuous laser was instable. The welded joint with good shape and free defects can be obtained with laser power of 2 200 W, welding speed of 55 mm/s and shielding gas flow rate of 10 L/min. The average tensile strength of joint was 388 MPa, reaching 63.98% of that of the base metal. The fracture of the joint was a mixed ductilebrittle mode.