Abstract: In order to investigate the seismic performance of a partially encased concrete(PEC)column fabricated with thin-walled steel plates-steel beam (reduced beam section)composite frame, a 1/2 scale test specimen of inter-story substructure was designed and a lateral-cycle repeated loading test was conducted. Based on the test observation and measurements, the failure mode and the hysteretic characteristic loops were derived, and the seismic performances including lateral stiffness, energy dissipation capacity and drift pattern were analyzed. Tests result show that reduced beam section can achieve the target of plastic hinge forming location away from joint to meet the seismic principle of strong joint. The displacement ductility coefficients of the inter-story and whole specimen are larger than 4.05 and 4.81, respectively, the maximum equivalent viscous damping ratios are 0.288 and 0.286. The lateral stiffness is evenly distributed along the height of the specimen, the drift pattern of the specimen shows the ideal inverted triangle pattern of the flexure-shear type. The plastic failure mechanism is induced by forming plastic hinge at all reduced sections, when the load-carrying capacity of the specimen is decreased to 85% of its ultimate strength,the drift angles of the inter-story and whole specimen are 0.043 3 rad and 0.039 4 rad which all surpass the limit(0.035 0 rad) inter-story drift at the maximum considered earthquake level, indicating that the specimen has superior seismic performance.