Using the improved strong field approximation method, the effects of long-range potential and short-range potential in above-threshold ionization energy spectra are investigated and the origin of the low-energy structure is also found. Meanwhile, in order to prove whether it is reasonable to use pure molecules instead of molecular ions in calculating the differential cross sections (DCS) of electrons and molecular ions, the DCS of electrons and atomic ions are calculated in some different incident electron energies conditions. The results of theoretical calculation and experiments show that the effect of long-range potential can be neglected and using pure molecules instead of molecular ions is reasonable with large-angle scattering in the large incident electron energies condition.