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.