The invention discloses a light darkening testing device for a rare earth-doped optical fiber, wherein the device comprises a visible light signal source, a passive matching optical fiber, a cladding light filter, a beam combiner, a tested rare-earth-doped optical fiber and a pump light filter which are sequentially connected; the input end of the beam combiner is further connected with a semiconductor laser; and a visible light beam emitted by the visible light signal source passes through the cladding light filter, the beam combiner, the measured optical fiber, the pump light filter and a wave filter and then enters a first power detector, and laser emitted by the semiconductor laser passes through the beam combiner, the tested optical fiber and the pump light filter and then is cut off at the position of the wave filter. According to the invention, the light darkening loss of a to-be-tested optical fiber at the laser working wavelength is indirectly calculated by using the power change after the visible light passes through the to-be-tested optical fiber, and compared with the prior art, the test can be completed in a shorter time. By arranging the cladding light filter and the pump light filter, cladding light from a signal source and pump light which is not absorbed and converted can be filtered, so that the test accuracy is improved.