The variation of temperature has an effect on maximal birefringent index of mica crystal, which accordingly influences the use precision of mica wave-plate. In order to reduce this negative effect and improve the precision of mica wave-plate under different temperature conditions, the thermal maximal birefringent index coefficients of mica crystal is measured by using the method of polarization interference spectrum. Firstly, with Daojing UV-3101PC spectrophotometer, the polarization interference spectrums of mica wave-plates with thickness of 80 μm, 300 μm and 813.5 μm are measured and the spectral drift is discovered at different temperatures. Then through accurate judgment of extreme points of the polarization interference spectrums, the maximal birefringent index of the mica wave-plate is calculated exactly. At last, expressions of the thermal maximal birefringent index coefficients for mica crystal are deduced at visible and ultraviolet bands. The experiment is performed under the condition of 0.05 nm wavelength resolution and the measurement accuracy of birefringent index can reach 10-5.