Abstract: Experimental validation of beam spectral source term parameters is one of the important procedures for boron neutron capture therapy (BNCT) physical dosimetry. It is crucial that the well-known the relevant information to precisely establish clinical treatment planning for correctly assessing the dose delivered to patients. To verify the reliability of the theoretically calculated photon energy spectra of source term, a new photon spectrometer applicable for BNCT beams (n/γ mixed beams with wide energy range and large radiation intensity) was designed. In order to realize the fast and accurate measurement of the photon spectrum for BNCT therapy beam, the radiation intensity should be reduced to prevent radiation damage and control the dead time of measuring system at a proper level, meanwhile, the neutron induced secondary photons must be suppressed as much as possible. The neutron fluence rate, photon count rate and secondary photon count rate proportion (secondary photon count rate/source term photon count rate) in the detector are optimized by Monte Carlo simulation, the radiation intensity at detector position is decreased largely, the secondary photon count rate proportion is reduced to 5.45. Furthermore, the calibration method of spectrometer response to different energy photons was also investigated at reference photon fields so as to obtain accurate response function for unfolding the photon spectra.