The thermodynamics behaviors of laser-induced plasma have been studied during CO2 laser welding on 6061 aluminum alloy with an optical multicultural analyzer and a high-speed camera. The plasma induced by laser in the initial stage and all the welding process have been analyzed in a stable welding process as the same as unstable welding process. The experimental results demonstrate that the deviation between electronic temperature and ionization temperature of plasma is very obvious in the initial stage during laser welding, then tends to balance gradually, in the same time, temperature gradient of plasma become smaller. In the stable welding process, there is only a small influence of laser power increase on temperature of the plasma, but the influence of the plasma′s size on weld area is important. When the plasma temperature increases sharply with the pressure increasing in the keyhole, the plasma will move up and down, it will induce in the interrupt of welding process or produce blowholes. Moreover, the size fluctuation of plasma will worsen weld shaping.