The present invention discloses a measuring refractive index device and a method thereof, which belongs to the field of optical detection. The device comprises a plurality of short coherent light sources with different wavelengths, a wavelength division multiplexer, an optical fiber coupler, two optical fiber collimators, a movable reflector, a reflector, a transparent slab, a wavelength-division multiplexer, a plurality of photoelectric receivers, and multistage optical fibers, and all components form a reference optical path and a measuring optical path. Through the cooperation of the reference optical path and the measuring optical path, the location of the cavity formed by the upper and lower surfaces of the tested transparent objects on the reflector and the transparent slab is measured, and with the distance between the reflector and the transparent slab, the thickness of the tested transparent object is obtained. The refractive index of the tested objective is obtained by comparing the optical path difference before and after the tested object is put in as well as the thickness of the tested transparent object. The measuring device provided by the invention is of simple structure, high measuring precision, able to conveniently measure the refractive index of planes and spherical mirrors and dispersion coefficient online with high precision.