In order to achieve the radial tolerance requirement in the process of machining, coating and aligning of the primary mirror of large aperture telescopes, we present a method based on image processing to realize the real-time detection of the radial eccentricity error of the primary mirror. Using optical imaging systems to take pictures of the reference hole which is fixed to the base of the primary mirror support structure and the inner hole of primary mirror while it is moving along the axis, the radial eccentricity error of the primary mirror with respect to the support structure is calculated by the edge extraction and the centroidal algorithm. Experimental results show that this method can realize the real-time measurement of the radial eccentricity error of the primary mirror, and the accuracy satisfies the radial tolerance requirement.