Abstract: In this paper,the seam tracking technology with non-contact ultrasonic sensor is introduced into gas shield arc welding.Because the protective atmospheres are different,the attenuation of the ultrasonic signal is different.Through experiments,the influences of the gases,such as Ar,CO₂ and Ar+CO₂,on the ultrasonic signal are studied.The results show that the signal attenuation is smaller in Ar and it is greater in CO₂.The seam tracking system with ultrasonic sensor can not work normally in CO₂ arc welding without adopting measures.Based on the theory analysis the main reason of ultrasonic signal attenuation in CO₂ is the relaxation absorption effect.It is difficult to eliminate the relaxation absorption effect in CO₂ arc welding seam tracking with ultrasonic sensor.Thus the physical isolation methods are advanced in this paper,and one of them is "a sleeve+a axial fan".It is useful for preventing CO₂ entering the space of the ultrasonic signal transmission.The results of experiments show that the non-contact ultrasonic sensor can be applied in CO₂ arc welding seam tracking,and the CO₂ arc welding seam tracking system with ultrasonic sensor can satisfies the requirements in real welding engineering.