Metal alloy composition changes will influence the metal mechanical properties, so quantitatively analyze the alloy composition during laser machining become very important。In this paper, the composition change of 7075 aluminum metal alloy was determined by calibration-free laser-induced breakdown spectroscopy during laser machining。The influence of different machining condition (laser pulse width) was also investigated。The experiment result shows that the concentration of magnesium in molten metal is lower than was present in the base metal。And the Mg loss increases with increasing pulse width of the laser machining。It can be seen that the selective vaporization of different elements are affected by the pulse width of laser machining.