Abstract: Using the model of solar irradiance spectrum during large solar flare and MSIS, the electron production rate during flare over sun-lit boundary region is calculated. It is found that during large solar flare the obvious increase of electron production rate over sun-lit boundary region occurs that, in turn cause the sudden increase of total electron content observed in this region. From the profile of electron production rate in different solar zenith angle, it can be seen that the largest electron production rate decreases with the zenith angle increasing and the heights of peak electron production rate increase. Calculation shows that the double peaks exist in the profile of electron production rate when the zenith angle less than 90° that correspond to ionospheric E and F region. Nevertheless, the peak value of electron production rate in F region is more larger than that in E region when zenith angle larger than 90°. Considering the feature of recombination of electron and ion in different ionospheric regions, it can be concluded that the sudden increase of total electron content observed by GPS mainly occurs in F region over sun-lit boundary region.