Abstract: Measuring the interference fringes of scientific and tracking targets simultaneously within an equal optical path angle in the atmosphere can result in the accuracy of astrometry at the micro-arcsecond level through precise optical path difference measurement between the two interference fringes. A phase measurement method based on spatial modulation is proposed for high-precision astrometry using the long baseline stellar interferometer. Synchronous phase shifting is achieved through polarization modulation, providing phase level optical path difference measurement. And the accuracy of optical path difference detection is further improved through multiple measurements and statistical averaging. This article demonstrates the feasibility of the phase detection method through numerical simulation and experiments, with detection accuracy better than 1/18 wavelength and optical path difference statistical measurement accuracy better than 5 nm. Furthermore, the source of error is analyzed through environmental disturbance measurement, laying a technical foundation for achieving the established scientific goal of China's under construction hundred meter long baseline interferometer.