The inherent discrepancy of birefingent dispersion relations of mica crystal has an unfavourable effect on designing wave-plate.A way of the polarization interference spectrum to study the dispersion of birefringent indexes of mica crystal is presented.The polarization interference spectrum of mica wave-plates are measured with spectrophotometer.Then through accurate judgment extreme points of the polarization interference spectrum,the maximal birefringent index at the extreme points can be calculated exactly and the dispersion curve is obtained from visible spectral band to ultraviolet spectral band.At last,expressions of the birefingent dispersion at any wavelength for mica crystal can be achieved through polynomial fitting data processing.This paper offers a accurate and simple method for measuring the birefringent index of mica crystal at any wavelength and temperature,also offers parameter choice basis for correctly using and designing crystal mica devices.