Abstract: Accurate calculation of the field strength variation throughout the whole reflection region when electromagnetic wave impinging vertically upon ionosphere is significant to the study of various non-linear effects generated in ionospheric modulation experiments. In this paper, the mathematical expressions of the electric field components of the characteristic waves are derived by the approach which couples the equation describing wave impinging vertically upon the ionosphere with the Forsterling equation, then the variation of each component of the electric field and the total electric field strength of the standing wave pattern under a specific density profile are calculated by the means of a uniform approximation throughout the region near the reflection point. The numerical results demonstrate that the total electric field strength of the O mode wave varies rapidly in space and several maxima generated below the reflection point, the swelling of the electric field strength is remarkable, and this effect is more obvious at higher latitudes than at lowers, also the geomagnetic field affects the variation of the wave pattern profoundly. While the electric field strength of the standing wave pattern of X mode wave has some growth below the reflection point, but its swelling effect is much weaker than the case O mode wave would be.