High power fiber laser was used to carry out laser cutting process experiments on 15 mm thick 2A12 aluminum alloy. The effect of defocusing amount, cutting speed, auxiliary air pressure, laser power and other parameters on cutting quality was systematically studied. In the research process, the cutting quality was judged based on the morphology of the section stripes and the morphology of bottom slag. First, through single-factor experiments, the optimal parameter range for defocus, cutting speed, auxiliary air pressure, and laser power is determined; then, the optimal process parameters are optimized by experiment, that is, the positive defocus and negative defocus are compared and selected. The best combination of defocus and cutting speed parameters. For 15 mm thick aluminum alloy laser cutting, the best process parameters are: laser power 9 kW, cutting speed 2 000 mm/min, auxiliary air pressure 1.8 MPa, nozzle diameter 3.0 mm, nozzle height 0.8 mm, and defocus +4 mm.