When　using　the　finite　element　method　to　analyze　seismic　soil-structure　interaction　(SSI)　problem　on　rigid　bedrock,　several　methods　have　been　developed　to　input　earthquake　on　the　lateral　boundary　of　finite　element　model.　These　boundary　conditions　include　the　free　boundary,　the　viscous-spring　(VS)　boundary,　the　tied　degrees　of　freedom　(TDOF)　boundary　and　the　free-field　loading　combined　viscous-spring　(FFL-VS)　boundary.　Their　accuracy　and　location　should　be　investigated.　In　this　paper,　the　formulations　and　ABAQUS　implementations　of　these　boundaries　are　given.　The　accuracy　properties　of　these　boundaries　are　then　compared　by　numerical　examples　including　the　free-field　and　SSI　problems.　The　comparison　studies　indicate　that　the　free　and　VS　boundaries　failed　to　reproduce　the　free-field　and　SSI　responses　when　a　relatively　small　size　SSI　model　is　employed.　The　TDOF　and　FFL-VS　boundaries　can　simulate　the　exact　free-field　response　and　have　good　performances　in　seismic　SSI　analyses.　The　FFL-VS　boundary　is　more　accurate　than　the　TDOF　boundary.　The　appropriate　locations　for　the　TDOF　and　FFL-VS　boundaries　depend　on　the　SSI　model.　A　simplified　approach　is　developed　for　appropriate　boundary　location.　This　approach　is　applicable　to　the　FFL-VS　boundary　but　not　to　the　TDOF　boundary　due　to　the　former　more　accurate　than　the　latter.