CsI photocathode is widely applied to high energy X-ray detection. And the spectral response is an important character of CsI photocathode. In this paper, the interaction process of high energy X-ray with CsI is analyzed and the spectral response of CsI photocathode is calculated in a 10-100 keV range. The influences of Compton scattering, X-ray fluorescence radiation and Auger emission on the spectral response are analyzed in accordance with the physical process of high energy X-ray interaction with CsI photocathode. These influences prove to be negligible in comparison with photo-ionization influence. Thus only the photoelectric transition is taken into account in calculation. According to the analyses of the processes of the photoelectron creation, transition and escaping, the formula for CsI spectral response is deduced as a function of secondary electron mean escape depth and photocathode thickness. The formula of secondary electron mean escape depth is then deduced as a function of X-ray energy. These formulae indicate that the mean escape depth of the secondary electrons increases markedly with the rise of X-ray energy and has a remarkable influence on the CsI spectral response. The spectral responses for different CsI thickness values are then calculated in a range of 10-100 keV. The results show that 1000 nm CsI has the best response under 20 keV, while 10000 nm CsI has a higher response over 60 keV. Then the calculation data are compared with experimental data of Hara's and Khan's hard X-ray streak camera measurements. These data agree well with each other and prove that our calculation of CsI spectral response for high energy X-ray is reliable. The spectral responses to CsI thickness for 17.5 keV and 60 keV are also calculated and shown in figures. These calculation data match experimental data of Frumkin and Monte-Carlo simulation data of Gibrekhterman. The measurement error of Frumkin's experiment and the uncertainty of the secondary electron mean escape depth are considered to be the reasons for the deviations of calculation and experimental data. The figures of spectral responses to CsI thickness also reveal the optimal thickness values of CsI for different X-ray photon energies. It is shown that 1 m is the optimal thickness for 17.5 keV X-ray detection, and 10 m is optimal for 60 keV. Finally the spectral response of CsI photocathode in a 10-100 keV range is calculated and the formulae prove to be reliable. According to these formulae and calculations, the optimal thickness of CsI photocathode can thus be given for designing and optimizing the high energy X-ray imaging detectors.