在最小偏向角法的光栅衍射实验中，现有最小偏向角的判断误差约与分光计上的望远镜最小分辨角相当. 为了减小该项判断误差，本文利用衍射光达到最小偏向角时与此时反射光束平行这一特点，借助直角反射镜改进了最小偏向角的判断方法，理论上可大幅度降低判断误差，同时采用对称测量法将测量误差进一步减小为单侧测量方法的一半，实验结果验证了该方法的可行性.

The judgment uncertainty existing in the least deviation angle method is about the same as the minimum resolution angle of the spectrometer’s telescope in the grating diffraction experiment. By using the characteristic that the diffracted light reaches the minimum deviation angle and is parallel to the reflected beam，the method of judging the minimum deviation angle is improved by means of orthogonal mirrors. Thus，theoretically，the judgment uncertainty could be reduced and the measurement error achieved in the symmetrical measurement method could be further reduced to half of that done by the single-sided method. The experimental result has shown that this method is feasible.