Co-phasing alignment of the optical multi-aperture is effective method in achieving high-resolution imaging. A correction method based on interference image sharpness is proposed to resolve the piston error between sub-apertures. A numerical simulation model is established and the correction range and piston error of the proposed correction method are analyzed according theoretically. According to the results, under polychromatic light, this iterative optimization method achieves a high-precision, closed-loop correction of piston errors smaller than the coherence length. In this method, the sharpness function of the far-field interference image is considered to be the objective function, and the integer multiple of the half-center wavelength is used as the step size for the bilateral perturbation gradient descent algorithm.