Ferritic/martensitic steel, with Cr atomic content in a range of 7%–15%, is a promising candidate for advanced nuclear power systems, due to its swelling resistance and creep fracture resistance under irradiation. Under thermodynamic conditions, Cr segregation usually occurs at grain boundary (GB) in Fe-Cr alloys. However, irradiation can greatly accelerate this process. The enrichment of Cr at GB will enhance precipitation, resulting in embrittlement; while the depletion of Cr at GB may greatly weaken the corrosion resistance properties. In the present work, thermodynamic segregation and radiation-enhanced segregation of Cr element at GB in Fe-Cr alloy is investigated by using the Wheeler-Boettinger-McFadden (WBM) phase-field model. The simulation results show that temperature has a great influence during thermodynamic segregation of Cr at the GB without radiation: when the temperature is lower than 500 ℃ the segregation amount of Cr at the GB is relatively small; when the temperature is higher than 500 ℃ the Cr concentration at GB increases significantly. In addition, as the concentration of Cr in the matrix increases, the amount of relative increase of Cr concentration at GB decreases. However, the Cr concentration at GB under irradiation is significantly enhanced, compared with the counterpart without irradiation. With the increase of dose rate, the Cr concentration in the center of GB also increases. Moreover, with the increase of Cr concentration in the matrix, the relative increase of the Cr concentration at the GB weakens.