The invention provides a large-aperture grating diffraction efficiency measurement device and measurement method. The device comprises a light source, a monochromator, a transmission fiber, a beam splitting mirror, a two-dimensional scanning mechanism, a sample table, a reference light detector and a test light detector. The diffraction efficiency value of single position of a large-aperture grating is calculated according to the ratio of the measured light intensity to the reference light intensity and the ratio of the diffraction light intensity to the reference light intensity. Then a fiber measurement head is scanned by the two-dimensional scanning mechanism in horizontal and vertical directions so that the diffraction efficiency value of each position of the large-aperture grating is obtained in turn and thus measurement of the diffraction efficiency of the large-aperture grating can be completed. Measurement of the diffraction efficiency of the large-aperture grating can be realized, the construction cost of the measurement system can be reduced, the safety hidden trouble in the measurement process can be eliminated, the measurement speed of the diffraction efficiency of the large-aperture grating can be substantially accelerated and the measurement data of the diffraction efficiency of the large-aperture grating are ensured to have great repeatability and reproducibility.