The　reinforced　concrete(RC)pier　is　subject　to　brittle　failure　when　strengthened　using　plain　ultra-high　performance　concrete(UHPC).　Three　rectangular　RC　pier　specimens　(one　specimen　is　not　strengthened,　and　the　other　two　are　strengthened　using　UHPC　with　built-in　reinforcement　mesh)　were　prepared　to　test　the　axial　compression　capacity　of　RC　piers,　and　with　the　initial　stress　levels　as　gauges.　Given　the　existence　of　secondary　stresses　in　the　strengthening　process　of　a　real　pier,　the　axial　ultimate　bearing　capacity　test　was　conducted　to　analyze　the　failure　process　and　variation　of　load　bearing　capacity　of　the　pier.　The　finite　element　models　of　the　specimens　were　developed　in　Abaqus　to　analyze　the　impact　of　thickness　and　compressive　strength　of　UHPC　on　the　axial　ultimate　load　bearing　capacity　of　the　specimens.　Based　on　the　test　results　and　finite　element　analysis,　simplified　equations　to　calculate　the　axial　ultimate　load　bearing　capacity　of　the　pier　strengthened　by　UHPC　are　derived,　in　the　case　that　the　thickness　of　the　UHPC　encasing　exceeded　the　threshold　thickness.　The　calculations　were　compared　with　the　results　of　the　test　and　numerical　simulation.　It　is　shown　that　compared　with　the　pier　that　is　not　strengthened,　the　axial　ultimate　load　bearing　capacity　of　the　two　piers　strengthened　using　UHPC　are　increased　by　173%　and　159%,　respectively.　The　axial　ultimate　load　bearing　capacity　of　the　UHPC-strengthened　piers　is　lowered　as　the　initial　stress　level　rises,　while　grows　as　the　thickness　and　compressive　strength　of　UHPC　increase.　The　proposed　equations　are　able　to　generate　calculations　with　higher　accuracy,　which　can　provide　theoretical　support　to　the　calculation　of　the　axial　ultimate　load　bearing　capacity　of　the　UHPC-strengthened　RC　pier　with　the　existence　of　secondary　stress.　©　2022,　Journal　Press,　China　Railway　Bridge　Science　Research　Institute,　Ltd.　All　right　reserved.