The ring cage is used to suspend an inertial mass and then detect the gravity with an extremely high sensitivity for electrostatic suspension space accelerometers. Geometric errors are required to be at submicron level, which makes it challenging and expensive to machine the ring cage by means of ultraprecision grinding. We propose an alternative solution by optical machining based on interferometric characterization of the geometric error. The flatness of the internal surface is measured with a skip-flat test. To deal with the mechanical perturbation in parallelism measurement, a differential method is proposed to simultaneously cancel out the figure error and tip-tilt of the cavity. Perpendicularity of the internal surfaces is indirectly characterized by combining the internal flatness, parallelism, and perpendicularity of the external surfaces. We introduce another interferometer to monitor the parallelism of two retro-reflectors in real time and measure the perpendicularity and pyramidal error of the external surfaces simultaneously. The test procedure and data reduction are illustrated experimentally.