<p>This study investigates the motion behavior of a slender flexible particle in a backward-facing step (BFS) flow using the direct-forcing fictitious domain method, with a particular focus on the trapping phenomena near the separation vortex region. Three distinct motion modes are identified: periodic rotation or oscillation within the vortex (trapping), downstream transport (escape), and transition state exhibiting unstable trapping. A dynamic balance among inward migration, centrifugal effects, wall interactions, and elastic forces enables the particle to achieve stable orbital motion within two distinct limit cycles. The topology of these orbits is governed by parameters, including the aspect ratio, structural flexibility, deformation intensity, and fluid inertia, all of which are characterized by the Reynolds number (<i>Re</i>). Specifically, fluid inertia plays a dominant role in facilitating particle trapping. At a fixed <i>Re</i>, a particle with a smaller aspect ratio tends to migrate inward and become trapped, whereas one with a larger aspect ratio is more likely to escape. Structural flexibility, especially when enhanced by confinement near the wall, leads to elastic deformation that induces secondary vortices and a weak flipping motion. The deformation intensity <i>α</i> significantly influences the lateral migration of the slender particle after the initial release; a larger <i>α</i> causes it to drift toward the channel centerline, increasing the probability of escape. These findings provide a theoretical foundation for optimizing the transport and capture of slender soft swimmers in complex flow environments.</p>