Squareness measurement is an important part of precision measurement a nd an important index of machinery production.Based on the theory of parallel beams′ far field inter ference and diffraction,a dark spot can be generated at the center of the laser beam coded by a vortex pha se plate in order to improve the squareness measurement precision.The center position of the dark spot is only decided by the center of the vortex phase plat e and is stationary when the laser beam drifts.The dark spot is used as a scanning spot in this method.The sq uareness measurement of the two guide rails is accomplished by recording the cen ter positions of the dark spot while scanning each guide rail.The X guide fitting linear equation is y＝0．1395x＋0．1428,and the fitting coefficient of R^2is 0.9994.The Y guide fitting linear euqation i s y＝－17．17x－0．06126,an d the fittin g coefficient of R² is 0.9999.The angle between two lines is 85.4°.T he squareness error is about 4.6°.The error is acaused by environmental vibrati on and verticality of CCD pixel cutting.Theoretical analyses and experiments hav e proved the feasibility of this method which has a larger scanning area and a m ore precise location than the traditional one.