The utility model relates to a device for measuring the diffraction efficiency of a reflective grating with an ellipsoidal reflector, which leads the ellipsoidal reflector into a measuring system. The ellipsoidal reflector is characterized in that all light which is emitted from a focus is led to a second focus. Lighting light wave and the grating are crossed at a first focus of the ellipsoidal reflector, and the probe of a detector is fixedly arranged at the second focus of the ellipsoidal reflector. Consequently, all the light of the lighting light wave which is incident into the grating in any angle can be received by the probe of the detector. The system can automatically receive diffraction light energy in a large angle in real time. The diffraction light energy can be detected automatically in a whole process, so that the space of the system is saved. The size of the measuring system is effectively reduced and the weight is reduced, and meanwhile, the real-time fixed-point measurement of the diffraction efficiency is realized. In addition, the device for measuring the diffraction efficiency of the reflective grating with the ellipsoidal reflector is applicable to the requirements of different detection angles and has very high universality only by generally adjusting the positions of the ellipsoidal reflector, an incident point of the grating and the probe of the detector.