To measure the spatial azimuths of different instruments in the upper and lower planes rapidly, an angle-measuring system based on magneto-optical modulation and a polarization splitter was proposed. An angle-measuring model of the system was deduced by describing the Jones vector of polarized light, and the influence of light source fluctuation on the angle-measuring precision was eliminated through signal processing method of “difference divided by addition”. Then, the relationships between the transmittance and the incidence angle, azimuth for the two pathways of optical signals from the Wollaston prism were analyzed, as well as their effects on the measurement results. Furthermore, the dependence of gain differences from optical signal attenuations, device drifts, and circuit performance of opto-electronic elements in two optical paths on the measurement precision were discussed. Finally, a magneto-optical modulation method was proposed to eliminate the difference of transmittances and gain coefficients of the two signals for the achievement of high-precision measurement. The experimental observation demonstrates that the measurement time is 15 s and angle-measuring accuracy is better than 5" within +8～-8°. These results show that the proposed method has some advantages on the fast angular measurement velocity, high precision, and so on.