Abstract: The ionospheric CT imaging is a limited angle tomography problem. The lack of horizontal rays severely degrades the vertical resolution of the ionospheric CT reconstruction results. Many people intuitively thought that the adverse effects due to the lack of horizontal rays can be compensated by including rays at low elevation angles. However, it is shown by both theoretical analysis and numerical simulation that the influence of rays at low elevation angles on the CT results is too small to mitigate the degradation in vertical resolution due to the lack of horizontal rays. The numerical simulations show that electron density images reconstructed are almost no changes whether consider rays at low elevation angles or not. But, when disregards 680 rays at low elevation angles and 680 rays at high elevation angles at same time, the change is very large. The CT imaging result consists with the model after taking out 680 rays at low elevation angles. But The CT imaging result changes largely comparing with the model and the small-scale structures cannot be discriminated when taking out 680 rays at high elevation angles.