The characteristic of high power microwave(HPM) propagation through ionosphere is mainly determined by the diversification of the atmospheric refractive index, while the refractive index has positive correlation with the electron concentration at corresponding propagation layer. The critical frequency of ionosphere in linearity condition and the atmospheric refractive index formula are derived. The electron concentration and the atmospheric refractive index are numerically simulated based on ionospheric electron concentration distributing model. The diversification of the refractive index and the refractive index gradient relating to the altitude are also analyzed, and experiment has been taken under low power microwave condition which validates that the corrected value is closer to the pr