A spectrometer method for determining the thickness of quartz plate along the crystal axis is presented based on the optical rotatory dispersion effect of the quartz. Its operating principle is analyzed by means of matrix formulation, and the result indicates that the thickness of the quartz plate along the crystal axis will be exactly obtained by accurate judgment of the transmission curve of a quartz plate sandwiched between two crossed polarizers. The error in the experiment is analyzed and it is found that the measuring accuracy will be improved by choosing longer wave band, lower scan speed, shorter response time and smaller slit width. At the same time, the theoretical analysis proves that the measuring accuracy of this method is higher than the digital micrometer's. In the end, two quartz plates with different thicknesses are measured by three different methods, respectively. It is shown that the accuracy of the tested thickness is 0.1 μm and is in good agreement with the theoretical prediction.