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.