Based on the test of four flat beam-column joints, a set of design methods for the inner and outer core areas of flat beam-column joints considering the steel fiber in the inner and outer core areas and the unbonded prestressed tendons in the flat beam are obtained through theoretical derivation. The method makes full use of the strength of concrete in the inner and outer core areas of the joint, correctly considers the role of column axial pressure, and carefully checks the vertical shear resistance of the joint. The reliability of the proposed design method is verified by the experimental data.