Beam drift is the uppermost factor that affects the accuracy of the propagating direction of a laser beam.It causes serious error in laser straightness or coaxiality measurement at a long distance.A novel differential compensation approach for laser beam drift is put forward in this paper.A beam splitter was used as a position-sentivie part.The first reflected beam from the splitter was detected using a four-quadrant detector.The second reflected beam from reverse direction by the splitter was d...