Clinically,　circle　of　Willis　(CoW)　is　prone　to　anomaly　and　is　also　the　predominant　incidence　site　of　intracranial　aneurysms　(IAs).　This　study　aims　to　investigate　the　hemodynamic　characteristics　of　CoW　anomaly,　and　ascertain　the　mechanism　of　IAs　initiation　from　the　perspective　of　hemodynamics.　Thus,　the　flow　of　IAs　and　pre-IAs　were　analyzed　for　one　type　of　cerebral　artery　anomaly,　that　is,　anterior　cerebral　artery　A1　segment　(ACA-A1)　unilateral　absence.　Three　patient　geometrical　models　with　IAs　were　selected　from　Emory　University　Open　Source　Data　Center.　IAs　were　virtually　removed　from　the　geometrical　models　to　simulate　the　pre-IAs　geometry.　For　calculation　methods,　a　one-dimensional　(1-D)　solver　and　a　three-dimensional　(3-D)　solver　were　combined　to　obtain　the　hemodynamic　characteristics.　The　numerical　simulation　revealed　that　the　average　flow　of　Anterior　Communicating　Artery　(ACoA)　is　almost　zero　when　CoW　is　complete.　In　contrast,　ACoA　flow　increases　significantly　in　the　case　of　ACA-A1　unilateral　absence.　For　per-IAs　geometry,　the　jet　flow　is　found　at　the　bifurcation　between　contralateral　ACA-A1　and　ACoA,　which　exhibits　characteristics　of　high　Wall　Shear　Stress　(WSS)　and　high　wall　pressure　in　the　impact　region.　It　triggers　the　initiation　of　IAs　from　the　perspective　of　hemodynamics.　The　vascular　anomaly　that　leads　to　jet　flow　should　be　considered　as　a　risk　factor　for　IAs　initiation.