Abstract: Using a coaxial monitoring system for detecting the optical emission in CO₂ laser beam welding,the time-field and frequency-field characteristics of the coaxial optical signals under different penetration statuses for welding of oblique plates were studied in this paper.The experimental results showed that under constant laser power and welding speed,the signal intensity varied with varying thickness and the signal intensity was maximum at the "moderate full penetration".In frequency field,the barycenter of frequency spectrum(FBC) of 2-6 kHz range was minimum at the "moderate full penetration".