B₄C film reflective mirror which is used in X-ray free electron laser beamlines may be damaged by ultra-high brilliance femtosecond X-ray lasers. In this paper, a theoretical simulation method based on Monte Carlo simulation is provided to predict the damage threshold of B₄C/Si-sub mirror irradiated by X-ray free electron lasers at photon energy 3-25 keV. Monte Carlo model based on Geant-4 was used for simulation of irradiation energy deposition distribution in the mirror. The melting points of mirror materials were taken as the criterion of damage threshold. The photon flux threshold at for designed grazing incidence angle in 3-25 keV photon energy range was calculated based on the simulation result and the theoretical model. According to the simulation result, the incidence flux threshold increases with the photon energy increasing. Also, most of the irradiation energy is deposited on silicon substrate and it’s recommend for predicting the damage threshold of the mirror based on the material melting point, not only the B₄C film but also the Si substrate should be considered.
 

B4C, damage threshold, X-ray free electron laser, Monte Carlo