The influences of the filler metal powder on the intensity threshold, weld formation and process stability have been studied during high power CO2 laser welding of aluminum alloy through the comparison between welding with and without metal powder. The experiment results demonstrate that the addition of the metal powder enhances the energy coupling efficiency during laser welding and reduces the power density threshold which has been validated by the testing data of Laserscope UFF 100.The power density threshold of A2219 can be reduce to about 50%. Meanwhile, the pictures collected by the high speed camera indicate that the addition of the metal powder weakens the fluctuation of the laser-induced plasma and keeps the plasma stabilizing above the workpiece. So a stable welding process and well-formed weld can be obtained in a relatively easy way.