The total cross section and differential cross section of high-energy electron impact excitation can be calculated with Born approximation. The differential cross section is proportional to the so called generalized oscillator strength. The target atom or ion may be excited to infinite bound states, auto-ionizing states and adjoint continuum states which can be treated in an unified manner by Multichannel Quantum Defect Theory. Thus, we can define the generalized oscillator strength density as the strength per unit excitation energy. Taking the lithium atom as an example, we present here a summary of the variations of the generalized oscillator strength density with respect to excitation energy as well as momentum transfer. Comparing with recent accurate experimental data, the validity of Born approximation is discussed.