Compared to the accurate model of Faraday anomalous dispersion optical filter(FADOF), the energy level of strong field model is simple, so it can be used to give some analytical expressions. However, it is only valid when the magnetic field is strong enough. By solving transmission spectrum of the FADOF for sodium D2 transition by the two models, and testing these results by the experimental data, applying condition for the strong-field model is successfully found. When the magnetic field is less than 0.1 T, the transmission spectrum difference of the two models is greater than 50%, and when the magnetic field is greater than 0.3 T, the difference is less than 5%.