Abstract: In this paper, the γ ray spectrum of the standard γ radioactive source (¹³⁷Cs, ⁶⁰Co) was measured by using a lanthanum bromide detector, and the experimental process was theoretically simulated by using the Monte Carlo program Geant4, and the response matrix of the γ ray spectrum in the energy range of 40 keV~3 MeV was generated. Using the Tikhonov regularization matrix method, the conversion relationship between the Geant4 simulated γ ray spectrum and the corresponding absorbed dose value is obtained, that is, the energy spectrum-dose conversion G(E) function. Based on the G(E) function, the corresponding air absorbed dose rate values were obtained by converting the experimental energy spectrum of the standard γ source ¹³⁷Cs, ⁶⁰Co and soil source measurements, and compared with the theoretical calculated values. The results show that the experimental results obtained by the G(E) function method are in good agreement with the theoretical calculated values, and the deviation is within 7.2%, which verifies the accuracy of the method.