Laser-arc　hybrid　welding　process　is　closely　related　to　the　arc　atmosphere.　By　using　a　laser　power　meter,　a　beam　quality　diagnosis　instrument　and　a　high　speed　camera,　the　beam　and　arc　characteristics　were　investigated　during　the　vertical　interaction　between　a　CO2　laser　beam　and　a　TIG　arc　in　argon　and　helium　atmosphere　comparatively.　The　experimental　results　demonstrate　that　the　laser　power　attenuates　dramatically,　the　beam　defocuses　and　the　beam　quality　is　seriously　worsened　while　the　arc　voltage　drops,　the　arc　column　expands,　and　even　combustion　wave　supported　by　a　laser　generates　in　argon　atmosphere.　However,　the　beam　and　arc　characteristics　seldom　change　during　the　laser-arc　interaction　in　helium　atmosphere.　The　difference　of　the　laser-arc　interaction　results　from　the　great　difference　of　the　electrons　number　density　due　to　the　difference　of　the　gas　ionization　energy.　The　electrons　number　density　in　the　helium　arc　is　10　times　less　than　that　in　the　argon　arc,　and　thus　the　inverse　bremsstrahlung　absorption　coefficient　of　the　helium　arc　is　two　orders　of　magnitude　lower　than　that　of　the　argon　arc.　Meanwhile,　there　is　less　difference　of　the　refractive　index　between　the　helium　arc　and　the　air,　so　there　is　no　obvious　refraction　of　the　helium　arc　to　the　laser　beam.