Abstract: The structure-borne acoustic emission (AE) signals during activating flux pulsed laser beam welding of stainless steel were detected in real-time. According to the AE signals, the characterization method of plasma flume information and the mechanism of activating flux welding were studied. The results showed that the TiO₂ activating flux improves the absorption to laser energy, which reinforces the energy intensity of plasma in welding process and increases the heat transfer to materials. The behavior of plasma was tested and evaluated by AE signals of plasma detected in activating flux welding process. The calculation about AE count of plasma AE signals, root mean square (RMS) waveforms and power spectrum distribution reflect the improvement of plasma energy intensity in activating flux pulsed laser beam welding.