The study investigated the dynamic motion simulation of a sail-assisted ship under service conditions with respect to different wind conditions. Energy-saving effect based on the automatic control was also assessed. The line-of-sight method is used to decide the desired heading angle and then the PD (Proportional-Derivative) controller is applied to manipulate the rudder to maintain the target course. Furthermore, the way of deciding the ideal sail turning-angle based on the aerodynamic characteristic curve is established to get the maximum wind thrust on the ship. The time domain trajectory of the ship motion is simulated by the 4^(th) order Runge-Kutta method. The mathematical model established here is useful for the prelimnary design of the automatic control system of sails on a ship and also helpful to the future application of wind energy on ships.