The invention relates to a measuring device for a large-sized optical fiber preform. The measuring device includes a light-transmitting container and matching oil, and a scanning laser light source isarranged on the outer side of the light-transmitting container and connected with a horizontal displacement device. A photoelectric position sensor is installed on the other side of the light-transmitting container corresponding to the scanning laser light source. The measuring device is characterized in that the photoelectric position sensor includes a polyhedral cylinder capable of rotating around the horizontal axis, photoelectric position sensing devices are mounted on at least two cylindrical surfaces of the polyhedral cylinder, one photoelectric position sensing device is mounted on each cylindrical surface, all the photoelectric position sensing devices are connected and arranged in a front-rear mode in the axial direction, and the photoelectric position sensing devices mounted onthe adjacent cylindrical surfaces are connected in a front-rear mode. The measuring device is reasonable, simple and convenient in structural arrangement and not only is large in measuring diameter, but also is low in manufacturing cost, and a function of adopting the small-sized photoelectric position sensing devices to measure geometric parameters, such as a refractive index profile and core andcladding diameter, of the optical fiber preform with a larger diameter is achieved.