The main refractive indices of calcite crystal are measured by the means of auto-collimation, and the thermo-optical coefficients are calculated. The coefficient expression of Sellmeier equation is obtained by solving Sellmeier equation strictly and the refractive indices of different wavelengths are calculated, which accord with experimental results very well. The measured main refractive indices of calcite at 488-nm wavelength are identical with the values obtained by Sellmeier equation.