Abstract: Mathematical model of rotating arc welding system is the theoretical basis of obtaining better welding quality, rotating arc signal processing and welding seam deviation extraction. The shape of weld pool has a great effect on the length of welding arc, thus affecting the accuracy of the mathematical model. Using double line structured-light vision sensor scanned the weld shape of weld ending stage and then reconstructed three-dimensional deposited metal shape of weld pool shape, selecting the simplex method to solve the length of welding arc. On this basis, the mathematical model of the rotating arc welding system and simulation model is established. The results showed that compared with the mathematical model based on triangular cone weld pool shape, the one based on the shape of the actual deposited metal of weld pool can effectively eliminate the current signal error caused by the deformation of the work piece during the welding process and decrease the error of weld rotation and weld seam deviation identification, thus the mathematical model of rotating arc welding is improved.