Spectroscopic ellipsometer is a common instrument for measuring film thicknesses and optical properties, whose accuracy largely depends on the calibration procedure of ellipsometry system. We propose a new calibration method by measuring several standard thin film samples with known thickness and dielectric function to obtain the experimental Fourier coefficient spectra. Fit these spectra to theoretical spectra which includes the calibration parameters using least-squares method and then solve these parameters of the ellipsometry system, including the azimuth of polarized elements, the retardation of waveplate, the azimuth of waveplate and the incident angle etc. We have extended this method to broadband spectrum of 200~1000 nm and verified the validity by measuring the SiO2 thin films in the thickness range of 3~13 nm and demonstrated good measurement accuracy of 0.194 nm. This new method is easier and faster than traditional calibration method.