The influence of assistant gases on the thresholds of laser induced plasma shielding was studied by using a 20 kW CO 2 laser processing system to weld low carbon steel. The results demonstrate that the critical power densities of laser induced plasma shielding for different assistant gases are in the order from low to high values as following: Ar→N 2→CO 2→He. The plasma shielding threshold could be as low as 1.85×10 6 W/cm 2 when argon was used as the assistant gas in our experiment. The influence of gas on the threshold of plasma shielding depends mainly on the thermal conductivity and dissociation energy of gas, while the influence of ionization enetgy is the secondary compared to the thermal conductivity and dissociation energy. The low power density of plasma shielding in argon is dominantly due to the poor thermal conductivity of argon which causes that the LSC wave can easily overheat and propagate.