To explore the technological characteristics in fiber laser cutting of medium-thickness aluminum alloy sheets, the fiber laser cutting of an 8 mm thick AA2219 Al alloy is carried out. The effects of process parameters such as laser power, cutting speed, defocusing distance and assistant gas pressure on the kerf quality are systematically investigated. The kerf quality is assessed by the dross height and the fraction of oblique striation zone of the lower part of kerf. The experimental results show that the kerf quality is mainly determined by laser power and assistant gas pressure. The dross height of the kerf is reduced to minimum when laser power increases to 5.4 kW and the range of gas pressure increases to 1100-1500 kPa. Moreover, in order to further improve the kerf quality of medium-thickness aluminum alloys, a simple Laval nozzle is designed and made by the hydrodynamics simulation based on the aerodynamics theory. The experiment with this nozzle discloses shows that the fraction of oblique striation zone is reduced from 0.5 to 0.14, while the dross height does not nearly change.