First-principles calculations based on density functional theory, using PW91 functional have been performed to study the adsorption of Alkali metal (AM) on C(100)(2×1) surface. The stable geometries, adsorption energies for all adsorption configurations have been calculated on half a monolayer and one monolayer. The preferred binding sites have been determined to be valley-bridge sites at the coverage of 0.5ML. At higher coverage of 1ML, two AM adsorbates were found to reside in pedestal site and valley-bridge site, respectively. Work function analysis showed that when AMs are adsorbed on C(100)(2×1) surface, the work function decreases linearly with increasing coverage and reaches a minimum at Θ=0.5ML. At higher coverage, the work function is increased again, which may be caused by depolarization effect of the adsorbate.