To measure the defects on the inner and outer surfaces of a capsule, a null micro-interference capsule surface-defect measurement system based on white-light scanning interferometry is proposed. White-light interference technology is combined with Linnik micro-interference technology and an optical path-matching module is introduced to measure the different surfaces of different diameter capsules. Based on the concept of spherical null interference, a spherical surface is introduced as a reference surface to expand the measurement field-of-view. In actual measurements, piezoelectric ceramics are used to scan a capsule surface vertically, thus achieving a full-field white-light interference image. Phase-shifting and bat-wing correction algorithms are used to restore the morphology of the capsule surface. The proposed measurement method is built to measure the standard step plate, and the measured values are consistent with the nominal values, thus verifying the effectiveness of the measurement method. Subsequently, a capsule is measured. The results show that the proposed measurement method can accurately measure the distribution of defects on the inner and outer surfaces of the capsule.