Fast Magnetosonic (MS) waves are right-hand polarized waves. They propagate both inside and outside the plasmasphere, and the wave vectors are almost perpendicular to the ambient magnetic field. Fast MS waves can lead to local electron acceleration, scattering of outer radiation belt energetic electrons, and scattering of energetic protons etc. Using the fully and high-density approximated dispersion relations, the dispersion curve of fast MS with different wave normal angle is analyzed, and the minimum resonant energy between MS waves and ions (H^{+}, He^{+}, and O^{+}) are calculated. The results show that, as the wave normal angle is relatively small, the minimum resonant energy obtained by the high-density approximated and fully dispersion relation is quite close to each other in a high density and weak magnetic field space plasma. Remarkable error occurs when the high-density approximated dispersion relation is used in the low and medium density space plasma or when the wave normal angle is greater than 88°. Therefore, the fully dispersion relation must be used in these cases.