A method for determining a conventional true value of air kerma [kinetic energy released per unit mass], comprising the following steps: Step 1: establishing a small-scale radiation field comprising a shielding box with a side length of not more than 1.5 meters; • providing an incident hole on a side of the shielding box for incidence of incident rays; • arranging a check point within the shielding box; • providing a test hole on the upper surface of the shielding box; arranging a dose feature point within the shielding box; • segmenting the shielding box into two parts by a plane perpendicular to the connecting line of the incident hole and the check point and that contains the check point, the dose feature point being located at a part close to the incident hole in the shielding box and at a position not directly irradiated by the incident rays; • arranging a gamma spectrometer in the shielding box, a reference point on a probe thereof being superposed with the dose feature point and the gamma spectrometer being fixed in the shielding box; Step 2: • selecting a proper radiation source and source strength and • providing incident rays for the shielding box Step 3: • selecting a plurality of gamma ray dosimeters as experiment for establishing a prediction model of the air kerma conventional true value at the check point; Step 4: • arranging a probe of an instrument to be detected at the check point • recording scattering gamma spectra measured by the gamma spectrometer and • introducing the scattering gamma spectra to the prediction model to obtain an air kerma conventional true value. Step 1: establishing a small-scale radiation field comprising a shielding box with a side length of not more than 1.5 meters; • providing an incident hole on a side of the shielding box for incidence of incident rays; • arranging a check point within the shielding box; • providing a test hole on the upper surface of the shielding box; arranging a dose feature point within the shielding box; • segmenting the shielding box into two parts by a plane perpendicular to the connecting line of the incident hole and the check point and that contains the check point, the dose feature point being located at a part close to the incident hole in the shielding box and at a position not directly irradiated by the incident rays; • arranging a gamma spectrometer in the shielding box, a reference point on a probe thereof being superposed with the dose feature point and the gamma spectrometer being fixed in the shielding box; • providing an incident hole on a side of the shielding box for incidence of incident rays; • arranging a check point within the shielding box; • providing a test hole on the upper surface of the shielding box; arranging a dose feature point within the shielding box; • segmenting the shielding box into two parts by a plane perpendicular to the connecting line of the incident hole and the check point and that contains the check point, the dose feature point being located at a part close to the incident hole in the shielding box and at a position not directly irradiated by the incident rays; • arranging a gamma spectrometer in the shielding box, a reference point on a probe thereof being superposed with the dose feature point and the gamma spectrometer being fixed in the shielding box; Step 2: • selecting a proper radiation source and source strength and • providing incident rays for the shielding box • selecting a proper radiation source and source strength and • providing incident rays for the shielding box Step 3: • selecting a plurality of gamma ray dosimeters as experiment for establishing a prediction model of the air kerma conventional true value at the check point; • selecting a plurality of gamma ray dosimeters as experiment for establishing a prediction model of the air kerma conventional true value at the check point; Step 4: • arranging a probe of an instrument to be detected at the check point • recording scattering gamma spectra measured by the gamma spectrometer and • introducing the scattering gamma spectra to the prediction model to obtain an air kerma conventional true value. • arranging a probe of an instrument to be detected at the check point • recording scattering gamma spectra measured by the gamma spectrometer and • introducing the scattering gamma spectra to the prediction model to obtain an air kerma conventional true value.