Abstract: The characteristic harmonic method was used to detect the deviation of rotating arc sensor.Through the mathematical analysis and simulation of arc length model, the relationship between the weld seam deviation, welding torch obliquity, the initial phase and the phase angle of tracking were studied.The characteristic harmonic method was improved by the conclusion that the real part and imaginary part can be used to detect the weld seam deviation and welding torch inclination, respectively.Also, in the case of only weld seam deviation is detected, the use of real part of characteristic harmonic is better than the amplitude method.The experiment results show the improved characteristic harmonic method has stronger anti-interference ability than that of the traditional characteristic harmonic method.