tTo understand the laser–matter interaction in fiber laser welding of aluminum alloys, the effects of laserpower on the characteristics of fiber laser induced plasma plume were studied by emission spectroscopicanalysis firstly. The plasma characteristic parameters including electron temperature, electron density,ionization degree, and inverse bremsstrahlung linear absorption coefficient were computed according tothe spectral data. It was found that the laser power of 5 kW is a turning point. After the laser power reaches5 kW, the plume changes from a metal vapor dominated weakly ionized plasma to a strongly ionizedplasma. The corresponding phenomena are the dramatic increase of the value of characteristic parametersand the appearance of strong plasma shielding effect. The calculation of effective laser power densitydemonstrated that the plasma shielding effect is dominated by inverse bremsstrahlung absorption. Thefinding suggested the plasma shielding effect must be considered in fiber laser welding of aluminumalloys, rather than is ignored as claimed in previous view.