Abstract: For Fizeau optical interferometric telescope, eliminating the relative phasing piston errors between sub-mirrors plays an important role in realizing high resolution imaging. The broadband spectrum fringe contrast detection method is effective to detect the relative phasing piston errors. In this paper, we analyze the relationship between piston errors and point spread function of system based on physical principles, and simulate the images of co-phasing far field fringe. Furthermore, we describe the experimental process of co-phasing detection and control, and the data preparation method. Besides we provide an algorithm of weighted contrast and a solution of maintaining co-phasing. Finally, co-phasing far field images are acquired and compared with simulation image. The experimental results show that power distribution and FWHM values of these two images are pretty close, and detection and control of phasing piston errors can be realized using this method.