In　order　to　study　the　mechanical　performance　of　UHPC　under　triaxial　compression,　the　conventional　triaxial　experiment　was　carried　out　to　analyse　the　mechanical　properties　such　as　the　failure　modes,　stress-strain　curve,　peak　stress　and　strain　by　considering　the　confining　pressure　and　steel　fiber　content　as　experimental　parameters.　The　results　show　that:　the　specimens　without　confining　pressure　and　steel　fiber　exhibit　splitting　failure,　while　the　others　exhibit　shear　failure.　The　elastic　modulus　and　elastic　segment　curve　shape　of　the　stress-strain　curves　remain　unchanged　as　confining　pressure　and　steel　fiber　content　change.　With　the　increase　of　confining　pressure,　the　peak　stress　and　strain　increase　continuously.　With　the　increase　of　steel　fiber　content,　the　peak　stress　increases　firstly　and　then　remains　unchanged　and　the　axial　peak　strain　increases　firstly　and　then　decreases,　while　the　hoop　peak　strain　increases.　Based　on　the　Drucker-Prager　two　parameter　criterion,　the　octahedron　failure　criterion　calculation　method　of　UHPC　was　established　by　analyzing　its　octahedron　normal　stress,　volume　strain　relationship　and,　its　octahedron　shear　stress　and　shear　strain　relationship.　©　2023　Tsinghua　University.　All　rights　reserved.