The invention discloses a device for measuring the diffraction efficiency of an intermediate infrared blazed grating, and the device comprises a tunable intermediate infrared testing laser, a guide laser, a polarization device, a polarization light splitting device, a to-be-measured optical grating, a three-dimensional electric displacement platform, an optical detector, an attenuation piece, and a spectrometer. An intermediate infrared optical grating causes the differences of first-order diffraction output power of intermediate infrared laser with different wavelengths, different incident angles and different polarization characteristics, so the device carries out the direct measurement of the diffraction efficiency of the intermediate infrared optical grating, and more truly reflects the actual diffraction efficiency of the intermediate infrared optical grating in actual application. The device employs polarization light-splitting measurement and double-optical-path detector comparison detection, and guarantees the measurement precision. The device can achieve the continuous measurement of the first-order spectrum diffraction efficiency of the intermediate infrared optical grating.