The deployment of ships into water involves complex multi-body dynamics and fluid–structure coupling phenomena, which are inherently interdisciplinary. To effectively analyze the motion and force variations of the parachute rope during the ship’s entry into the water, the calculation of the ship’s water immersion is divided into three phases. In the airdrop phase, the 9-DOF multi-body dynamics equations governing the parachute system of the ship are formulated to determine the forces acting on the parachute and the ship’s attitude. In the water entry phase, a numerical model of ALE fluid–structure coupling is developed for the ship’s entry into the water, with LS-DYNA employed to calculate the attitude and water load of the ship during and after entry. Finally, the motion trajectory of the ship during the immersion phase is substituted into the 9-DOF dynamic equation derived in the airdrop phase to obtain the force curve of the parachute rope throughout the immersion phase. The work presented in this paper offers valuable insights for the comprehensive motion analysis and parachute rope strength validation of the airdrop system for ships.