The invention relates to an X-ray energy spectrum measurement method based on flat crystal diffraction imaging. By using the method, disadvantages that energy spectrum resolution is poor and measurement efficiency is low in an existing method are overcome. The method is suitable for multi-energy X-ray continuous energy spectrum measurement below 100keV. The method comprises the following steps of 1, estimating an energy spectrum scope of an X ray to be measured and determining a diffraction crystal parameter and a measurement system parameter; 2, carrying out imaging on all the diffraction angles in a diffraction angle scope and recording a diffraction image and a transmission image; 3, for a crystal determined in the step1, using a direct current X-ray source and an energy distinguishing detector to measure a rocking curve of each energy X ray to a diffraction crystal in the energy spectrum scope to be measured and calculating an integral diffraction coefficient; 4, according to the transmission image in the step2, calculating diffraction energy corresponding to each pixel of the diffraction image in the step2 so as to acquire an X-ray diffraction energy spectrum; and combining the integral diffraction coefficient of each energy to be measured in the step3 to calculate an incident X ray energy spectrum.