The　internal　forces　of　a　four-row　three-span　building　with　and　without　base　isolation　were　compared　in　this　paper.　The　influence　of　parameters,　such　as　the　load　of　isolation　layer　＇　s　top　beam-slab,　the　stiffness　of　it　and　beam　-　column　stiffness　ratio　of　upper　structure,　were　considered　in　the　research.　It　is　found　that　the　end　moments　of　the　first　floor　＇　s　columns　in　base　isolation　structure　increase　remarkably　compared　to　that　of　non　-　isolated　structure　,　the　end　moments　of　the　beams　on　the　rubber　bearings　are　proportional　to　the　loads　acted　on　it　.　Meanwhile,　a　method　to　calculate　the　internal　force　of　base　isolated　structure　under　the　vertical　load　is　proposed　based　on　non-isolated　structure　and　a　new　concept　of　isolation　influencing　coefficient　is　present,　in　which,　the　internal　forces　of　base　isolated　structure　are　obtained　by　multiplying　that　of　non-isolated　structure　and　the　corresponding　isolation　influencing　coefficient.　On　the　other　hand,　the　influence　of　some　structural　parameters　and　load　to　the　factors　was　also　investigated　quantitatively,　and　the　formulas　to　calculate　these　factors　were　given.　Method　proposed　in　this　paper　is　consistent　with　that　for　horizontal　load　calculation.　So,　it　improves　the　design　theory　of　base　isolated　structure.