According to Faraday effect and lock-in amplification principle, the magneto-optical modulation of polarized light through an optical material occurs after deflection. The output light contains a fundamental signal and a frequency-doubled signal whose frequencies are equal and doubled to modulation frequency respectively. Based on the analyses of amplitudes of fundamental and frequency-doubled signals, we put forward an algorithm for measuring the rotation angle. According to the relation between optical rotation angle and magnetic declination, we put forward a method to calibrate the magnetic declination in the Faraday magneto-optic effect. The feasibility of such measurement approach has been verified by our experiments.