Beam diagnosis was used to measure the laser radius and transmit direction when the CO₂ laser passed through the DC TIG arc and the influence of laser defocus and the arc position laser on the spot were studied, and the reflection angle was calculated. Results showed that Facula radius was enlarged for 0 and+10 mm laser defocus because of arc plasma's "Negative Lens F.fl'ect",and facula radius was reduced while 1~10 mm defocused. Since arc's refraction as a "Prism",laser's trana- mission direction was deflected to cathode for a small angle of 0. 80. Facula was elliptical for laser in- cidenee near are cathode.