<p>Multistage helical structures exhibit remarkable mechanical stability with bending and twisting flexibility, leading to widespread applications in both natural and engineering fields. The global responses of multistage helical structures under complex bending loads are essential for the prediction of service life. This study develops a theoretical model based on differential geometry and thin rod theory to characterize the elasto-plastic mechanical behavior of multistage helical structures subjected to bending loads. By incorporating the periodic geometric relationships, the explicit formulations of bending stiffness and equivalent bending curvature of the multistage helical structures are derived. A three-point bending experiment is conducted for validation, and theoretical results agree with experimental data. Compared with the classical Costello model, the present model exhibits improved agreement between theoretical predictions and finite element (FE) simulations in characterizing the global mechanical response of helical structures. This study examines the influence of laying angle variations on mechanical properties, demonstrating that reducing laying angle enhances structural flexibility by reducing bending stiffness. The elastic-plastic response of the first-stage and second-stage helical structures under bending loads is further obtained. The energy dissipation of helical structures during loading and unloading is studied, and the results indicate that energy dissipation decreases with reducing laying angle. For the complex application of helical structure, the elastic-plastic bending behavior under combined bending and tension loads is investigated. This study offers insights for the optimized design of multistage helical structures subjected to complex bending loads.</p>