Abstract: To explore characteristics and rules of weld formation of aluminum alloy by high power fiber laser welding, 3mm-thick 5052 aluminum alloy was welded by laser with Gaussian and flat-topped beams respectively. The influence of weld formation characteristics, laser power and scanning speed on weld shape was analyzed comparatively. The results show that flat-top beam welding is mainly of thermal conductivity mode, and weld pool is U type, which is easy to produce pores, cracks and other defects. Gaussian beam welding has 4 modes. 4 modes are transformed mutually with the change of laser power and scanning rate. Weld pool is丁type under deep penetration mode. At scan rate of 20mm/s, the lower and upper critical power density of effective deep penetration welding of Gaussian beam are 8.8×10⁵W/cm² and 9.2×10⁵W/cm². With laser power of 2.7kW, the lower and upper critical power of effective deep penetration welding of Gaussian beam are 77J/mm and 90J/mm. Under the premise of the required welding power density, line energy can be used as the criterion of laser deep penetration welding. Line energy and power density play decisive role in welding mode and pool shape.