Large-scale SiC lightweight mirrors are usually designed as non-uniformity rib-structures in the rear face for reducing weight. Both the optical face and the rib of the mirror are so thin that deformation of the profile caused by grinding force cannot be neglected. The electrolytic in-process dressing (ELID) of grinding wheels attains specular finishs on brittle materials, with high efficiency and surface roughness on the nanometer scale. In this study, the profile deformation of a SiC work during grinding processes was investigated by ELID grinding experiment and FEM simulation method. Especially, in order to finish large-scale SiC lightweight mirrors with high form acuracy, a new method combined ELID-grinding and machining simulation was proposed.