The response sensitivity of CsI photocathode is an important parameter for the quantitative diagnosis of X-ray spectroscopy by soft X-ray streak camera, and its theoretical calculation has important guiding significance. The current theoretical analytical models are based on the one-dimensional random walking model of secondary electrons generated by thin film photocathodes, including the Henke model under the condition of normal incidence of X-rays and energy greater than 1 keV, and the Fraser model under variable angle incidence and photocathode thickness greater than 100 nm, which have certain limitations. In this paper, the basic expression of the probability of secondary electron transmission is introduced, and the general expression of the response sensitivity of CsI photocathode is deduced, which varies with X-ray energy E, photocathode thickness t, and angle θbetween X-ray and cathode surface in a larger parameter range (X-ray energy 0.1–10 keV, photocathode thickness 10–200 nm). Finally, the theoretical calculation results of this paper are discussed by comparing it with those from the Henke model, Fraser model, literature data and experimental data on Beijing synchrotron radiation facility, which verifies the accuracy and universality of the computational model, and a theoretical reference is provided for optimally designing the CsI photocathode in high-time-resolution spectrometric quantitative measurement.