In　the　ductility-based　seismic　design　of　reinforced　concrete　(RC)　bridges,　the　equivalent　plastic　hinge　length　(Lpeq)　of　the　RC　pier　is　critical　in　determining　the　ultimate　deformation　capacity　of　bridge　piers.　However,　developing　an　equivalent　plastic　hinge　model　for　RC　double-column　piers　(DCPs)　is　challenging　due　to　uncertainties　regarding　boundary　conditions　and　dynamic　axial　forces.　This　article　aims　to　investigate　the　Lpeq　of　RC　circular　DCPs.　The　quasi-static　cyclic　loading　tests　of　six　RC　DCPs　were　firstly　carried　out　and　the　effects　of　dynamic　axial　forces　on　the　Lpeq　as　well　as　the　ultimate　deformation　in　DCP　were　then　systematically　investigated.　The　test　results　show　that　plastic　hinges　develop　at　both　the　top　and　bottom　of　the　piers,　with　noticeable　differences　in　section　curvature　due　to　the　boundary　conditions.　Based　on　these,　the　boundaries　of　the　effects　of　dynamic　axial　forces　were　then　established　based　on　statistical　methods.　An　equivalent　plastic　hinge　model　of　the　RC　DCP　was　developed　and　the　Lpeq　prediction　equations　for　the　top　and　bottom　of　the　pier　were　obtained　using　a　genetic　optimization　algorithm,　based　on　the　double-bending　assumption.　Comparison　with　experimental　data　shows　that　the　proposed　model　accurately　predicts　Lpeq.　The　research　results　can　provide　a　reference　for　the　seismic　design　of　RC　bridges　with　DCPs.