Abstract: To address the issues of rapid stiffness degradation and significant residual deformation in concrete-filled steel tube columns under seismic action,a novel steel-FRP composite bar (SFCB) reinforced ultra-high performance concrete-filled steel tube (UHPCFST) composite column is proposed.The seismic performance of UHPCFST composite columns was investigated through quasi-static tests and finite element models,analysising the effects of concrete strength,steel tube yield strength,longitudinal reinforcement type,reinforcement ratio,stirrup ratio,axial compression ratio,and slenderness ratio.The results indicated that all composite columns exhibited compression-bending failure.Compared with UHPCFST composite columns with conventional steel bars,those with SFCB showed slightly lower bearing capacity and energy dissipation,but greater deformability,reduced residual deformation,slower degradation of stiffness and bearing capacity,and better repairability.Increasing concrete strength,reinforcement ratio,or steel tube yield strength,or decreasing the slenderness ratio effectively improved the bearing capacity and initial stiffness of the composite columns,whereas increasing the stirrup ratio had limited impact.When the axial compression ratio increases from 0.15 to 0.25,the load-bearing capacity of the composite column increases accordingly.As the axial compression ratio further increases to 0.35 and 0.45,the load-bearing capacity significantly decreases,the P-Δ second-order effects become more pronounced,and the stiffness degradation rate increases.Finally,a restoring force model for SFCB-UHPCFST composite columns was established based on a degraded trilinear model.The predicted results aligned well with the test results,reflecting the hysteretic characteristics of the SFCB-UHPCFST composite columns accurately.