The dynamic nature of the marine environment induces six-degree-of-freedom ship motions, posing challenges to the safety of maritime operations. This paper presents a 6-PUS parallel ship motion simulation platform designed to replicate ship motion characteristics under heavy-duty conditions in a 5th sea state. The kinematic and dynamic models of the motion simulation platform were established using the closed-loop vector formulation and the Newton-Euler method. To achieve the platform’s 5th sea-state simulation capability, a novel evaluation method based on the effective workspace coverage ratio was proposed to assess its motion performance. Combined with load-bearing capacity metrics, the dimensional parameters of the mechanism were optimized. The design was validated to meet the requirements for 5th sea-state simulation. To address heavy-duty driving demands, an electro-hydraulic hybrid drive system was adopted. This system counterbalances the mass of the platform assembly and payload under static conditions, significantly reducing the dynamic forces on the motor-screw modules during motion. The accuracy of the kinematic and dynamic models was verified through simulations using RecurDyn software. This research advances the application of parallel robotic mechanisms in maritime engineering, providing a robust solution for high-fidelity, heavy-duty ship motion simulation.