Abstract: One kind of the CO₂ arc welding seam tracking system with non contact ultrasonic sensor is presented in this paper.The fuzzy-P control theory is applied in this system.Because the control rules and parameters have most bearings on the control characteristics of the system,the control rules and parameters must be optimized to improve the system characteristics.Firstly,the fuzzy-P control principle and the self-tuning fuzzy control rules are introduced.Then,the control parameters,such as fuzzy control scale factors α₁,α₂ and output scale factor K,are studied and defined by means of the computer simulation.And the threshold e_v of fuzzy-P control is defined.The results of the experiments show that the optimized rules and parameters of fevuzzy-P control by means of the computer simulation are feasible and effective.The CO₂ arc welding seam tracking system with ultrasonic sensor which fuzzy-P control is adopted can meet the need of practical welding seam tracking task.