The major purpose of this paper is to reduce the laser directional deviation of laser designator on a moving platform. A new method of inhibiting the laser beam positional error caused by platform movement and vibration is proposed. In this method, quadrant detector(QD)and fast steering mirror are combined to measure the angle between laser designator axis and the line-of-sight of the target, then a control signal composed with the angle errors is generated to aim the axis of the laser designator at the target steadily. This is a real time processing method and it is suitable for airborne laser-guided weapons with second-class guiding time.