For the sparse-optical-synthetic-aperture telescope system, the co-phasing detection plays an important role for the realization of system's high resolution in solving out piston errors between sub-apertures. In this paper, the relationship between piston error and system's far-field intensity was analyzed based on physical principles, then the impact of piston error on system's far-field images was clarified theoretically. Furthermore, an innovative co-phasing detecting method based on the far-field images' similarity was approved for the bi-sub-aperture systems, with its feasibility, detecting deviation and dynamic range were researched by simulation. The results proved that, with this method, the piston error between the system's two sub-apertures can be soundly detected principally while immune from the problems of 2 ambiguity and direction decision that might exist within some other co-phasing detecting methods. Plus its dynamic range shown from these results, this new co-phasing detecting method provides effective references for deeper research of co-phasing detecting techniques.