Abstract: The thickness of the inherent filter layer is one of the important performance parameters of the X-ray reference radiation device, and it needs to be quantitatively measured. This research is based on the new version of ISO 4037-1 standard specification, taking N series radiation quality (N-20-N-350) X-ray reference radiation device as an example. Through the PTW30013 ionization chamber, the radiation field under the secondary aperture of different sizes is measured to establish an experimental environment that meets the requirements. The results show that the thickness of the inherent filter layer of the reference radiation device is 0.122 mmAl measured by the half-value layer method; the equivalent inherent filter layer thickness of 4 mm Al under N-40-N-350 radiation quality is obtained by increasing the thickness of the aluminum filter. Using the conversion relationship between the mass-energy absorption coefficient of elemental aluminum metal and beryllium metal at different energies, the equivalent intrinsic filter layer thickness of 1 mm Be under N-20-N-30 radiation quality is obtained. This study provides a certain reference for the measurement of the inherent filtering of the X-ray reference radiation device.