Abstract: Eliminating the relative Optical Path Difference (OPD) between two sub-apertures in synthetic aperture telescope is a precondition for high resolution image. The fringe detection method is effective to decrease the relative OPD. The fringe angle depends on the position distribution of sub-aperture. The precise angle of fringe is an essential prerequisite for taking line sample along the normal direction. And then, on the basis of the maximum contrast value, the position corresponding to the minimum OPD between two sub-aperture can be obtained. Here, a method for calculating the angle of interference fringes based on Fourier transform and threshold traversal of image binaryzation is proposed. Firstly, the basic principle of the algorithm is introduced. Secondly, with the data for fringe angle of 43 degrees as the simulation data to test this algorithm, the result of 43.007 8 degree is obtained. The value of error is 0.018% compared with theoretical simulation value. In the end, contrast curves from two cases of rotating camera and non-rotating camera are compared. It is more benefit to get the accurate minimum OPD by the method of rotating camera to make the fringe along the longitudinal axis of camera according to the angle of fringe.