Abstract: The experiments were carried out with CO₂(slab) laser. The five stages of the inter actions between laser and powder in the laser welding with powder were analyzed.The influences of the filler metal powder on the laser power density threshold value, weld formation and process stability were studied during CO₂ laser welding of aluminum alloy. The experiment results demonstrate that the addition of the metal powder enhances the energy coupling efficiency and reduces the power density threshold value during laser deep welding. Such as the reducing rate is about 50%for A2219 alloy. The experiment results also indicate that the powder can enter the weld pool availably only the powder was delivened to the suitable area after laser beam at the surface of a part. The proper quantity of the alloy powder must match with the right power density and energy input,which can assure to obtain good weld appearance. Meanwhile, the addition of the powder can reduce the fluctuation of the laser-induced plasma and maintain its stability effectively in the laser welding. So a stable welding process and good weld appearance can be obtained in the experiments.