Abstract: The mechanical properties of a Buckling-Restrained Brace (BRB) after a fire and its theoretical analysis model are the main topics of this research. Firstly, a coupled temperature-force numerical simulation method for the mechanical properties of post-fire BRB is proposed, and the BRB’s post-fire mechanical properties under different factors are parametrically analyzed. Then, a macroscopic phenomenological continuum stiffness mechanical model for the post-fire BRB is established, and the model parameters are calibrated. Finally, the stiffness, yield point and, hysteretic energy of the post-fire BRB by the theoretical analysis model and numerical simulation are compared. The results show that the degradation of BRB’s performance after a fire is mainly affected by the restraint ratio and core width-thickness ratio. After a fire with 90 minutes, the energy reduction rate increases by 16.1% with the decrease of the restraint ratio, while the constraint ratio decreases by 8.3% with the decrease of the width-thickness ratio. Compared with the numerical simulation results, the average errors in the predicted skeleton curve stiffness, yield point and, single-turn hysteresis energy by the theoretical analysis model are 2.1%, 3.8%, and 6.8% respectively. The research may offer a reference for the post-fire seismic performance assessment of a building structure with the BRBs.