Abstract: To study the failure pattern and seismic performance of PEC column (partially encased concrete composite column)-steel beam inner-frame joints, three PEC column-steel beam inner-frame joints and one steel frame beam-to-column inner-frame joint were tested under low-cycle reversed loading. Several parameters including the failure mode, hysteretic behavior, load carrying capacity, ductility performance, degradation of strength and stiffness and mechanical mechanism of the inner joints were analyzed. The effect of axial load ratio and beam section variation on the seismic performance of the joints were studied. Experimental results indicated that the hysteresis curve of PEC column-steel beam inner-frame joints was of a spindle shape, and these joints had similar mechanical characteristics to steel frame joints. The initial stiffness and capacity increased about 40% and 31.1% respectively, while steel column filled with concrete. The specimens still had good ductility performance and energy dissipation ability. As the test axial load ratio increased from 0.25 to 0.35, the bearing capacity and the energy dissipation capacity of the specimens increased while the ductility performance decreased. The failure feature of specimens are the bending of beams and no obvious change in the degree of damage. The capacity and ductility performance of specimens were improved when the beam section on one side of the column were changed. The energy dissipation capacity, strength and stiffness degradation have no noticeable changes due to the variation in the beam section, but the failure patterns of the inner-frame joints were shearing diagonal compression in joint zones. The main reason for the failure patterns was the increase in the input shear force in the joint with the variation of the beam height on one side of column. The variable section beam joints designed according to the conventional joints cannot meet the seismic design requirements of the "strong node weak member", and it should be taken into serious account in engineering design.