In order to measure the mechanical angle repeatability of the variable-included-angle plane-grating-monochromator used in soft X-ray spectro-microscopy beamline at Shanghai Synchrotron Radiation Facility,a new method was proposed based on the phase plate diffraction collimation. This system was designed combining laser diode fiber with phase plate diffraction collimation technique. A 2-dimension CCD was adopted to collect the images,which was used to measure the displacement of the light spot,thus the angle repeatability of the plane mirror and the grating could be determined. The results show that this method could be used to measure the monochromator′s angle repeatability in a higher precision estimated to be ±0.1″,which is better than that measured by the commercial ELCOMAT 3000 auto collimator used in the same condition.