In this study, we analyze the changes in the spot morphology of the far field of the subaperture under the influence of gap and decenter of mask and explore the feasibility and applicable conditions of using the ideal circular aperture template library to match the far-field pattern generated by circular templates with gap and decenter for co-phasing. Further, the shape of the far-field is numerically simulated under different gaps and decenters of mask. The usage of ideal template narrow-band co-phasing is numerically simulated, and the co-phase accuracy of different piston errors under different gaps and decenters of mask is analyzed. The results prove that the far-field extension range will increase with the increasing gap. The larger the decenter of mask is, the less obvious the far-field effect modulated by the piston error will be. The ideal co-phase template can be directly adopted when the gap scaling factor or decenter scaling factor does not exceed 0.3. In this study, we expand the application of the narrow-band co-phase algorithm and provide the basis to further study this algorithm.