A device for on-site measurement of concentration of uranium in high temperature molten salts is provided. More particularly, this device can be directly applied to a pyroprocess for reusing spent nuclear fuel and determine concentration of uranium 3+ and 4+ chemical species using ultraviolet-visible light absorption spectrometry. The device includes first and second optical waveguides submerged in molten salts including uranium through a port formed at an upper side of a pyrochemical process apparatus; a lengthwise driver installed at the port to be operated to adjust a distance between the optical waveguides; a light source for supplying light to the second optical waveguide as any one of the optical waveguides; and a spectrometer connected to the first optical waveguide as the other one of the optical waveguides to analyze the light emitted from the second optical waveguide and introduced through the first optical waveguide via the molten salts.