Combining quadrant detector and fast steering mirror,a new measuring method of the positional error of laser beam on moving platform is proposed.In this method,the fast steering mirror is used to stabilize axis direction of outputting laser beam and the quadrant detector is used to measure the corresponding change of return laser position,then the coupling action between fast steering mirror and the movement of tracking platform is separated in order to get the angle between laser axis and line-of-sight of target.According to this,it can be concluded that the axis of the laser at the target steadily and guide the missile attacking precisely.This method is suitable for real time processing and it is suitable to the laser guided weapons with second class guiding time at last stage trajectory.