Abstract: The invention relates to a method and system for measuring the internal refractive index of an optical fiber preform with high precision. The internal refractive index of an optical fiber preform (2) is measured from the state of bending of a light ray (4) passing across the optical fiber preform (2), so that the refractive index distribution of the optical fiber preform (2) is found on the basis of the angle of bending of the light ray calculated from a specific relation between a light ray start position (8) and a light ray detection position (9). The surface of a light source (1) is scanned by a knife-edge (6) and the light ray start position (8) on the light source is determined depending on the interception of the light ray (4) by the knife-edge (6) or the de-interception of the light ray, thereby learning the start position (8) of the detected light ray (4) on the light source (1).