By means of CCD and A/D, flash speotrograph can be used to obtain spectral energe distribution of an instantaneous illuminant. Through the analysis for the energe relation of flash speotrograph, it is expounded in this article that how the output of flash speotrograph ia affected by the relative diffraction efficiency of grating, with examples of flash xenon lamp spectra in the wavelength range from 0.36μm to 10μm.