In　order　to　obtain　the　ductility　a　seismic　performance　of　reinforced　concrete　box　piers,　the　equivalent　plastic　hinge　length　and　ultimate　displacement　calculation　methods　for　reinforced　concrete　box　piers　were　discussed　based　on　bi-axial　quasi-static　tests　of　14　piers　and　moment　curvature　analyses　of　reinforced　concrete　box　cross-section　under　action　of　bi-axial　load.　The　results　showed　that　the　piers　with　larger　height-width　ratio　have　bigger　plastic　damage　area,　meanwhile　their　ultimate　curvature　drops　obviously;　the　calculated　yielding　displacement　and　ultimate　displacement　are　close　to　the　actual　measured　ones,　so　the　ultimate　displacement　calculation　method　can　be　used　in　ductility　a　seismic　analysis　for　reinforced　concrete　box　piers.