Background:　Anatomical　variations　of　the　circle　of　Willis　(CoW)　are　closely　associated　with　the　occurrence　of　intracranial　aneurysms　(IAs).　However,　the　impact　of　anatomical　variations　on　the　rupture　risk　of　IAs　remains　unclear.　The　purpose　of　this　study　was　to　explore　the　effect　of　artery　absence　on　the　internal　flow　and　rupture　risk　of　IAs.　Methods:　A　one-dimensional　(1D)　solver　was　combined　with　a　3-dimensional　(3D)　fluid-structure　interaction　(FSI)　model　to　effectively　quantify　the　hemodynamic　characteristics　inside　IA　under　artery　absence.　Results:　The　1D　results　showed　that　the　absence　of　anterior　cerebral　artery　A1　segment　(ACA-A1)　or　posterior　cerebral　artery　P1　segment　(PCA-P1)　will　trigger　a　compensatory　blood　flow　effect,　leading　to　significant　blood　flow　variations　of　the　anterior　communicating　artery　(ACoA)　and　internal　carotid　artery　(ICA).　By　FSI　calculation,　in　the　absence　of　ACA-A1,　the　maximum　wall　shear　stress　(WSS)　within　the　ACoA　aneurysm　increased　by　103%　or　more　compared　to　the　complete　CoW　due　to　blood　inflow　jet.　In　addition,　WSS　increased　by　45%　and　12%　in　the　contralateral　ICA　and　posterior　communicating　artery　(PCoA)　aneurysm　respectively,　whereas　it　decreased　by　33%　and　35%　in　the　ipsilateral　ICA　and　PCoA　aneurysm,　respectively.　The　absence　of　PCA-P1　had　a　less　significant　impact　on　the　global　blood　flow　of　the　CoW　compared　to　the　absence　of　ACA-A1,　but　it　still　led　to　an　increase　in　WSS　within　the　ipsilateral　ICA　and　PCoA　aneurysms　(25%　and　22%,　respectively).　Conclusions:　The　absence　of　ACA-A1　or　PCA-P1　may　serve　as　an　IA　rupture　risk　factor.　If　ACA-A1　or　PCA-P1　absence　is　identified　clinically　alongside　an　aneurysm,　proactive　treatment　strategies　are　advised.　©　AME　Publishing　Company.