Currently, offshore alongside replenishment is the main mode of ship replenishment, which operates by transporting goods via constant tension highline cables erected between receive and supply vessels. In this paper, a dynamic model of offshore alongside replenishment is established based on multibody dynamics theory. The system consists of ships, cargo, pulleys and highline cable. Rigid body assumption is adopted for ships, cargo and pulleys. The highline cable is modeled using the geometrically accurate beam theory. A contact model between pulley and highline cable is established based on Hertz theory. In order to improve calculation efficiency, a simplified force element model is proposed according to characteristics of constant tension cable, and its accuracy is verified by comparison with the exact model. The dynamic simulation of whole replenishment operation under complex sea conditions is carried out by using the simplified model, and the effects of ship motion, replenishment distance and tension on the transporting process are analyzed. The results show that safe operation can not be guaranteed under harsh sea conditions, long replenishment distance or too small tension.