Abstract: Limited by the characteristics of the boron neutron capture therapy (BNCT) treatment beam, it is difficult to measure the γ ray energy spectrum of the source term. There are few relevant work has been carried out. Up to now, only the γ energy spectrometer directly measured the source term γ rays, but the spectrometer was not been calibrated and unfold. In order to obtain an accurate γ energy spectrum of the source term, it is necessary to calibrate the γ energy spectrometer and establish the reliable response function of the system. This paper briefly introduces the γ energy spectrometer designed for BNCT treatment beam, and mainly introduces the calibration method suitable for this spectrometer. The spectrometer consists of a high purity germanium (HPGe) detector and a shield system. Since the entire spectrometer is designed for high intensity neutron/γ mixed fields, the shield effect of the spectrometer is so effective, and it is impossible to calibrate the γ response of the entire spectrometer system through experimental methods. By analyzing the spectrometer system, the dead layer distribution of the HPGe detector is the main influence factor, and other structures can be accurately obtained through X-ray radiography and geometric measurements. Therefore, the calibration method of the spectrometer is as follows: obtain an accurate model of the HPGe detector by X-ray radiography and calibration; use precision measurements, fine geometric measurement and material analysis to obtain an accurate model of the shield, and establish a calculation model of the spectrometer; acquire the response function of the spectrometer system through Monte Carlo N particle transport code (MCNP) calculation, and the energy spans from 0.1 MeV to 1 1 MeV with an energy interval of 10 keV. Among them, the HPGe detector dead layer distribution can be confirmed using the following steps. The efficiency of the HPGe detector was calibrated utilizing standard γ sources and high-energy γ reference radiation fields. The dead layer distribution of the HPGe detector was adjusted using MCNP to ensure agreement between calculated and measured efficiencies, thereby determining the geometric distribution of the dead layer. Therefore, using a combination of Monte Carlo simulation and experimental methods, the results can be traced to the corresponding national or national defense standards, ensuring the accuracy and reliability of the key parameters of the spectrometer, and laying the foundation for subsequent spectrum unfold work.