Concave diffraction grating not only possesses the function of dispersion and focusing for the light beam at the same time, but also has the advantages of aberration correction, low stray light, none ghost line existence and high signal-to-noise ratio. It has been received extensive attention in the spectral instrument field. As one of the most important performance properties of concave diffraction grating, the measurement level of the diffraction efficiency for concave grating becomes one of the most concerned topics in the spectral instrument industry. Double monochromatic structure is usually adopted to measure the diffraction efficiency for concave grating in the traditional method, but it involves two major problems: the first one is the differences of output bandwidths during measuring standard mirror and the tested grating, and the second one is the overlapping of diffracted spectra and scatter light in the instrument. Therefore, the traditional method which contains above problems influences the measurement accuracy of diffraction efficiency for high performance concave grating. In this paper, a new measuring method of diffraction efficiency based on Fourier Optics principle is proposed. The mathematical model of diffraction efficiency measurement is firstly deduced and then verified with ray tracing and Fourier optics simulation. Aiming at reducing the influence of the moving cube corners tilt error, lateral shift error, source instability and maximal moving distance error, we put forward to introducing an assistant detector to collect data together with the main detector. The relative mathematical deduction and simulation have been made under the condition of existence and non-existent of the assistant detector, the result shows that we can effectively control the influences of the above errors and can greatly increase the measurement accuracy of diffraction efficiency when the assistant detector is introduced in the light path. Compared with the traditional double monochromatic structure, our method not only solve the problem mentioned above, but also has the advantages of multi-wavelengths measurement in the same time, high luminous flux, high spectral resolution, and high wave-number accuracy.