Abstract: The various penetration status in deep penetration laser welding is classified.The welding condition and the weld bead formation of "weld pool penetration" and "moderate full penetration (keyhole penetration)" are described.A system for coaxial monitoring the penetration status in CO₂ laser welding is established.The influence of welding process variables and penetration status on the coaxial optical signal is studied in laser welding of planar plate.The experimental results show that as the heat input increases,the variation slope of the signal intensity is maximal when the penetration status changes from "weld pool penetration" to "moderate full penetration".