Based on 3%～5% relative errors of retarder for λ /4 mica waveplate and different absorption coefficients for the light oscillating along the fast axis and slow axis of mica waveplates, a polarimetric calibration system of the extremum method was proposed, and the influence of both waveplate retarder and its different absorption coefficients on the output light intensity was analysed. Precision measurement results of waveplates in six channels were obtained. The polarized degrees for the ideal linear polarized light and non polarized light were calibrated on the basis of the above results, and the relative errors were less than 1%.