When the transfer loading and unloading of the ship， the longitudinal displacement between the ship and the dock or between the ship is changed. The traditional method is to compensate for the change of displacement by adjusting the cable. In this thesis， the author a compensation method that the relative displacement between the ship and the dock as the input quantity， calculates the control quantity of the hydraulic system through the PLC control system， and compensates the change of the ship's longitudinal displacement by the way of the stroke cylinder expansion of the hydraulic arm.