Marine risers are important structures in marine engineering. In order to ensure their normal operation, this paper proposed a method that uses weak fiber Bragg grating sensing cables to complete shape reconstruction of the riser and uses the finite element simulation experiments to verify it. Firstly, the sensing cables were installed along the axis of the riser in the form of a backpack-style pipeline. Based on the Frenet-Serret framework, an algorithm for reconstructing the shape of the riser was designed. Then, a finite element model for monitoring risers was constructed, and strain data inside the sensing optical fiber cable various deformation conditions were extracted by solving the model. By combining algorithms, the three-dimensional shape of the riser was reconstructed. Finally, an analysis and calculation of the shape reconstruction error were performed. The results indicate that the end error of the designed riser shape reconstruction method was controlled within 1.7%. The method has a good effect on the reconstruction of the riser shape, with a simple monitoring form and certain engineering application value.