Abstract: To improve the accu racy of phase retardation measurement, a wave plate measurement method was proposed based on magneto-optical modulating and base frequency signal detecting. Firstly, a magneto-optical modulator was placed at a suitable location of the optical path to be adjusted and to make only even harmonic components be left in the measurement signal. And then, detecting the residue of the base frequency component instead of whole spectrum signal, the phase retardation measurement of wave plate was carried out accurately. Finally, the theoretical formulae and corresponding wave plate measuring system were established based on Jones matrix. The analysis results show that, when the ambient temperature changes 0.1°, the measurement uncertainty of the system can be less than 5'. Half-wave plates (HWP) and quarter-wave plates' (QWP) measurement results show that, the measurement standard deviation is about 2'.