The core of a multi-storey building is usually semi-enclosed because there are beams connecting two opposite wall edges at the height of the floors. The behavior of semi-enclosed cores differs from that of open cores especially in the aspect of torsion resistance because the coupling beams prevent warping of the core section. Due to this resistance, the coupling beam is subjected to a large amount of force, soon reaches the bearing limit state, and has a large deformation. Therefore, people have constructed diagonal reinforcement for the coupling beams to increase the ductility and workability of the structural system. Nonlinear models for ordinary beams do not fully describe the working of beams with diagonal reinforcement, which when used will affect the results of core structure analysis. The article will build a model of the post-elastic working coupling beams with the inclusion of diagonal reinforcement in accordance with the actual working of the beam to analyze the core structure with the case of the core structure that was established by J.S Kuang et al. calculated post-elasticity according to thin-wall theory and continuous-discrete model to verify the model. The analysis results show that, after applying the proposed coupling beam model, the core-vertex moment-rotation curve not only has a rule consistent with the theoretical calculation results but also analyzes the plastic flow and stiffness reduction of each beam instead of calculating in each area of beams as theoretically calculated.