We propose here a simple theoretical method to calculate excitation cross sections for electron-ion collision processes. At low energies, we apply the Multichannel Quantum Defect Theory to calculate excitation cross sections based on accurate data of energy levels which have been deduced from extensive spectroscopic measurements. At high energies, we then calculate cross sections based on Bethe Theory. At intermediate energies, we can apply interpolations to calculate mean cross sections if we smooth out local resonance structures. Reliable data for electronic impact excitations of ions are useful in the controlled thermalnuclear reaction researches. As an example, we calculate the (2S-2P) excitation for lithium-like ions to illustrate our proposed theoretical method.