The invention relates to a spectral reflectivity measuring device, which comprises an optical unit, a controlling unit and an ultrasonic ranging unit. Four optical transmitting channels are arranged in the optical unit, two upper channels adopt a cosine scatterer as an optical window, and two lower channels adopt a K9 material optical glass window. The optical window, an optical filter and a plano-convex lens are arranged at one end of the optical unit body, a sealing gasket is arranged on the both sides of the optical filter and the plano-convex lens, and a photosensor is arranged at the focal point of the other end of the plano-convex lens of the optical unit body. The controlling unit comprises a sensor, an amplifier, a digital/analog converter, a single-chip microcomputer, a U disk, aliquid crystal display and a keyboard, etc. The ultrasonic ranging unit consists of an ultrasonic sensing module based on the piezoelectricity principle. The invention achieves that the reflected light of the plant canopy and the radiating light of the sun are measured synchronously, the operational procedure of the measurement is simplified, and the measurement height is kept consistent automatically.