Objective Based on the lanthanum bromide scintillator detector, the calculation method of G(E) function was developed to measure the air absorbed dose rate. Methods Firstly, the gamma energy spectrumof the lanthanum bromide detector were simulated and the response functions with different energies were determined with Monte Carlo simulation method. Then, the G(E) function was calculated by the least square method. Finally, the air absorbed dose rate measured by the lanthanum bromide detector using G(E) function conversion method was compared with the theoretical calculation value based on the point source experiments. Results The experimental verification results showed that the relative deviation between thecalculated value with G(E) function and the theoretical calculation value wasmostly controlled within ± 6%, which verified the accuracy of G(E) function. Conclusion The results showed that the method of G(E) function could be applied to calculate the gamma radiation dose rate based on the in-situ the gamma spectrum with LaBr₃ detector.