The technique of free abrasive polishing with elastic restraint was proposed, and the work piece surface was polished by abrasive in suspending liquid with polishing tool as flexible restraint. The spherical tool machining area model reflecting the process influence factors, and the mechanical model of sphere and pyramid particles completely were established. The material removal mechanism was discussed and the feasibility of polishing free surface was analyzed subsequently. The experiments of polishing the optical glass use of two kinds of abrasive Al2O3 and SiC were carried out respectively. The validity of machining the ultra precise surface with flexible restraint abrasive was indicated, and the influence regulation of abrasive dimension was proved by the experimental results.