The　use　of　ultrahigh-strength　steel　in　civil,　bridge,　and　offshore　engineering　is　increasing　rapidly.　In　this　study,　the　seismic　behavior　of　a　concrete-filled　steel　tubular　(CFST)　column　base　joint　with　implanted　ultrahigh-strength　reinforcing　bars　has　been　investigated.　The　steel　tubular　and　the　implanted　reinforcing　bars　are　made　of　ultrahigh-strength　steel.　Four　column　base　joints　with　square　and　circular　cross-sections　and　with　0　and　0.25　axial　force　ratios　have　been　tested　to　discuss　the　hysteretic　capacity,　deformation,　ductility,　failure　modes,　and　energy　dissipation.　The　axial　force　is　the　main　parameter　to　improve　the　flexural　capacity　of　the　column　base　joints.　Owing　to　the　slip　between　the　infilled　concrete　and　the　steel　tubular　columns,　the　hysteresis　loops　show　an　obvious　pinch　phenomenon.　The　tested　specimens　demonstrate　good　ductility　behavior.　Finally,　the　flexural　capacity　of　the　CFST　column　base　joint　has　been　determined　by　analyzing　the　failure　characteristics　and　stress　of　the　CFST　columns.　The　flexural　capacity　of　the　CFST　column　base　joints　determined　by　the　calculation　formulae　matches　well　with　the　test　results.　The　ratio　of　the　calculated　and　tested　flexural　capacity　is　between　0.967　and　1.059,　and　the　absolute　relative　error　is　between　0.131%　and　5.595%.　©　2020　Elsevier　Ltd