Abstract: By using a coaxial monitoring system for detecting the optical emission in CO₂ laser beam welding, the frequency-filed characteristics of the coaxial optical signals under different penetration statues are studied. In frequency field analyzing, the FBC (barycenter of frequency spectrum) is presented to describe the characteristic of the frequency spectrum of the plasma optical signal. The experimental results show that as the laser power increase or welding speed decreases, the FBC of 2-6 kHz range jumps up to higher frequency when the penetration status transits from "moderate full penetration" to "weld pool penetration".