Abstract: Aiming at the key technical issues of the reference radiation field size of isotropic sources, GB/T 12162 series of GB standards stipulates the size of the reference radiation field when using an isotropic source. However, there is no specific regulation for the size of the irradiation under collimation. To reduce the influence of scattering in the gamma reference radiation field when used for radiation detection or monitoring instrument verification and value calibration, Monte Carlo simulation was carried out to explore the effect of the size change of the irradiation chamber on the energy distribution and dose rate value during the radiation source collimation. The boundary conditions of the collimated gamma radiation irradiation chamber were obtained, and the details of the gamma reference radiation field boundary research method and related standards are established and improved. The study provides a new method or approach for the size design of the irradiation chamber under the collimated irradiation state.