To solve the problem that the measurement precision of traditional azimuth measurement system based on sine wave magneto-optic modulation polarized light is low, a new model of measuring azimuth is presented to improve the measurement precision. The principle of traditional azimuth measurement system based on sine wave magneto-optic modulation polarized light is analyzed. As the signal from modulator is expressed with the omitted high-order term Bessel function expansions, the method error and truncation error are detected. A new model which does not include theoretic error is presented, and the new model expressed in simplex trigonometric function is established. Simulation results show that the system measurement precision of the new model is obviously higher than the traditional one. It provides a certain reference to apprehend the system principle and improve the system measurement precision.