Large deflection deformation of a steel strip is an important part of structural deformation detection. Deformation detection has been achieved using ultra-weak fiber Bragg gratings (FBGs), and the deformation reconstruction inversion algorithm of the tangent recursive algorithm has been analyzed theoretically. The strain distribution of the steel strips (2.5 m × 0.050 m × 0.002 m) with large deflection under different loads was obtained by pasting eight ultra-weak FBG sensors with spacing of 0.3 m on the surface of the strip. The steel strips deformation was inversed by tangent recursive algorithm, and deformation error under loading condition was analyzed by finite element simulation. The inversion value is compared with the theoretical value and the measured value, which proves the effectiveness and accuracy of the ultra weak FBG method proposed in this paper.Results show that the large deflection deformation of a steel strip under different loads can be reconstructed using the tangent recursive algorithm, and the relative errors of the coordinates of the sampling points are less than 6.21%. This paper provides a reference for the detection of structural deformation.