Abstract: In order to study the phenomenon that a ferry boat drifts athwart at the deserted crossing, a numerical simulation is carried out for the motion of a boat under the water flow action. Conclusions are as follows: The boat can keep a balance both along and across the water. But it is an unstable balance along the water, it will be lost because of an external disturbance, and the boat can not restore to its homing position again. While the boat is in a stable balance across the water, it can restore to the homing position again even if the boat loses the balance under an external disturbance.