In order to examine the seismic mechanism of a new partially encased concrete(PEC)column(weak axis)-steel beam(reduced beam section)composite frame, a 1∶2 specimen of the inter-story substructure was designed and built. A test of the specimen was conducted with lateral low-cycle repeated loading. Based on the test observation and data analysis, the seismic performance of the specimen, including the failure process, failure mode, hysteretic property, lateral stiffness degradation, energy dissipation capacity, and deformation pattern, was analyzed. Test results show that the failure mode is primarily induced by the plastic hinge forming in the reduced beam section. The displacement ductility coefficients of the inter-story in the pushing and pulling direction are 3. 91 and 3. 97, respectively, and the maximum equivalent viscous damping ratio is 0. 359. The drift distribution of the specimen shows the ideal inverted triangle pattern of the flexure-shear type. When the load-carrying capacity of the specimen is decreased to 85% of its ultimate strength, the inter-story drift ratio is greater than 4. 9% and the minimum connection rotation is 0. 366 rad, which all surpass the lower limit(1/30)of inter-story drift at the maximum considered earthquake level, indicating that the specimen has high collapse prevention capacity.