High maneuverability and energy efficiency are critical for underwater robots performing complex engineering tasks, and biological swimmers provide rich inspiration for achieving these capabilities. A key challenge lies in the design of an efficient and controllable propulsion mechanism. This study presents a novel bio‑inspired solution in the form of a highly flexible, controllable bistable nonlinear mechanism that functions as an artificial fishtail. The proposed design combines an elastic spine with a lightweight parallel linkage, enabling tunable bistability and precise shape control through active manipulation of the spine endpoint. This approach realizes, for the first time, a precisely controllable bistable elastic propulsion system, offering a promising pathway toward agile and energy‑efficient underwater robotic swimming.