A simple, practical, and effective method for measuring the rotation-angle-accuracy of the wavelength scanning mechanism in a Variable Included Angle Plane Grating Monochromator (VAPGM) was proposed in an ultra-high vacuum environment. The method was achieved by using a special angle mirror and a high-precision photoelectric autocollimator. Firstly, the relationship between angle and wavelength was described on the basis of the principle of VAPGM wavelength scanning. Then, the dependence of rotation-angle-accuracy on system resolution was analyzed. Finally, the principle, devices and steps of the method were introducted in detail.By using this method,the rotation-angle-accuracies of the Plane Mirror (PM) and the Plane Grating (PG) were measured, and the results show that they are 0.19″and 0.22″,respectively, which fulfills the technical requirements of VAPGM. The resolution power of this monochromator was tested by an ionization chamber after measuring the rotation-angle-accuracy, and the result is superior to 10000. The experimental result further verifies that the method is effective.