As the medium of azimuthal information, modulated polarized light can be used to transmit azimuthal information, and this technique can be widely applied in military, aerospace and biomedicine. The principle of measuring azimuth with magnetooptic modulation polarized light is expatiated on, and the error sources by existing methods are analyzed. In order to improve the measurement precision theoretically, the method of calculating azimuth directly is brought forward. The extremum of modulated light signal is extracted, and the equation between the extremum and azimuth is established. The equation′s solutions are analyzed, and the formula of calculating azimuth is acquired. At the same time, the measuring range of azimuth is wider because the double angle formula is introduced. The precision of the new method is compared with those of the existing methods. The results show that the theoretic measurement precision of the new one is higher and measuring range is wider. It provides a new way to measure spatial azimuth with high precision.