Using density functional theory, based on the pseudo-potential plane wave basis set, the geometries and  electronic structures of Li/Si(001) systems with different Li atom coverages were investigated systematically. The effect of Li adsorption on surface properties was also investigated. Our results indicated that Li atoms preferred to adsorb on high symmetry sites between adjacent Si-Si dimers and that the smallest average adsorption energy was predicted for the 0.75 monolayer (ML) coverage. By analyzing the band structures, the Si(001) surface varied from semiconductor to conductor, then to semiconductor again with increasing Li coverage. The bandgap of the Si(001) surface increased obviously at 1.00 ML coverage because of the significant destruction of surface Si-Si dimers after Li adsorption. Since electrons obviously transferred from the Li atom to the substrate, the work function of the surface decreased and oscillated with an increase in coverage. Furthermore, according to the calculated surface formation energy, the phase corresponding to 0.75 ML coverage should be difficult to observe.