To enlarge the transmission scale and improve the transmission precision of an azimuth transmission system based on sine wave magneto-optic modulation, a new method of transmitting the azimuth in a large-scale and high-precision was established. The principle of the current azimuth transmission system was analyzed, and a double-angle formula was introduced to enlarge the transmission scale. A model to measure the azimuth was established according to analyzing the relationship between the azimuth and the extremum of the modulated light, and the measurement model was expatiated by comparing the extremums. A integrated method combined the approximation in little-scale with the looking-up table in large-scale was presented to calculate the arc tangent function in the model. Simulation results indicate that the theoretic transmission scale is widened obviously and the precision is improved. Experiment results show that the actual transmission scale is -64-64° and the transmission error is under 10″, which is better than those of the current methods. The method provides a new way to transmit the spatial azimuth in large-scales and high-precision.