To solve the difficult problem of swing suppression of slender-beam payloads under rough sea conditions, the Multi-Tagline Parallel Anti-swing System (MTPAS) is applied to the lifting of slender-beam payloads in the offshore environment. The lifting dynamic model of the slender-beam payload under ship-motion excitation is presented using Newton's classical mechanics with the MTPAS dynamics and double-pendulum dynamics, and the dynamic characteristics of the double-pendulum system when subjected to ship-motion excitation are studied. The accuracy of the double-pendulum dynamic model and control strategy is verified by hardware experiments. The results of the study present an original idea to establish a dynamic model of a double-pendulum for transporting slender-beam payloads under rough sea conditions.