Abstract: To enhance the aseismic performance of reinforced concrete column-steel beam composite structure (RCS) beam-column joints, this study proposes a novel replaceable-composite stepped RCS beam-column joint for RCS composite structures. This new joint is formed by the synergistic interaction of two components: the replaceable shear energy dissipator (RSED) and the stepped flange connector (RSFC). Following the validation of a quasi-static test on the joint, simulations under quasi-static cyclic loading were conducted separately for the RSED and RSFC. A comparison of the hysteresis curves, of the skeleton curves, of the cumulative energy dissipation and, of the stiffness degradation of various specimens under different parameter conditions led to the identification of the optimal energy-dissipating cross-sectional forms for both the RSED and RSFC. Finally, the optimally configured replaceable-composite stepped RCS joint was assembled and its seismic energy dissipation performance was compared with that of a traditional RCS beam-column joint. The research results indicate that the optimal energy dissipation and design dimensions of the X-shaped shear keys within the RSED are directly related to the section reduction parameter. In the RSFC, the opening/closing angle θ, the rotational spacing d in the connection zone, and the length L of the buckling energy dissipation segment significantly influence the specimen's energy dissipation capacity. Compared to the traditional RCS joint, the application of the proposed replaceable-composite stepped RCS joint reduced the equivalent plastic strain (PEEQ) in the RC column and the von Mises stress in the RC column's rebar by 74.2% and 17.5%, respectively. At the same inter-storey drift angle of 0.04 rad, the new RCS joint’s cumulative energy dissipation increased by 43.9%, demonstrating its excellent energy dissipation capacity and reparability.