Fourier phase is important information for optical synthetic-aperture (OSA) imaging. The observed object′s phase is located in interferogram fringes. Factors affecting the OSA interferogram fringes′ initial phase were analyzed in detail from the mechanism of interferogram fringes in the image plane. With the original point value and the fringe peak displacement, the initial phase can be obtained from the fringes. With closure phase method, the error phase caused by atmosphere disturbance unnormal of star light direction to baseline direction, and telescope mechanic system, can be eliminated, and the real object Fourier phase can be obtained at last. With a threshold method in the frequency domain, noise affecting fringe peak′s location can be eliminated. The role of closure phase method is talked in the image reconstruction with hybrid iteration method.