A method is described of measuring absolute spectral response for Au and CsI transmission photocathodes in soft X-ray streak camera, which is of great importance for the inertial confinement fusion (ICF) diagnostics. Transmission photocathode is conventionally employed as photo-to-electron conversion accessories. To derive quantity information of X-ray spectra, the absolute response of photocathode must be calibrated in a range of interest. Here Au and CsI transmission photocathodes with slits are calibrated respectively on Beijing Synchrotron Radiation Facility (BSRF), in a photon energy range of 60 eV5500 eV. This method has an uncertainty less than 10% and good feasibility. Calibration results are in good agreement with the calculation results obtained from the Henke's photon emission model, with CH substrate effect revised.