This study investigated a method of measuring the absorbed dose rate of γ-ray in air using a Φ7.5 cm×7.5 cm NaI(Tl) detector. Simulation of the NaI(Tl) detector using the Monte Carlo method was performed to obtain the γ-ray energy spectrum of the detector in the energy range of 50 keV~2.5 MeV. The G(E) function was used to calculate the air-absorbed dose rate of radioactive sources. When solving the G(E) function, the influence of the K_max and the optimization factor M on its calculation of the air-absorbed dose rate was considered, and the solution of the G(E) function was optimized to improve the calculation accuracy. The relative deviation S of the air absorbed dose rate calculated by the optimized G(E) function from the theoretical standard value was less than ±1%. Finally, after the comparison of experimental measurements, it was found that the relative deviation between the calculated air absorbed dose rate of the G(E) function and the measured results of the dose rate meter was less than ±10%, indicating that the optimized G(E) function value of the NaI(Tl) detector can be used for the application of air absorbed dose rate measurement.