Abstract: The groove angle, workpiece deflection, root gap and tack weld of the longeron, which easily cause the inaccurate or difficult identification of the weld deviation. In order to solve this problem, a deviation identificaion method in longeron automatic welding was proposed, based on laser displacement sensor. By establishing a arc length model, and making use of the linear relationship between the different harmonic components of arc length, a weld deviation solution method was presented, which could eliminate the effect of groove angle and workpiece deflection on weld deviation identification. A container longeron was taken for experimental research to identify weld deviation, it has been shown that the method proposed in this paper has good anti-interference capability to root gap and tack weld, which provides a new theoretical and practical reference for deviation identificaion in longeron automatic welding.