The threshold displacement energy of Fe and Fe-Cr-Al alloys and the influence factors were investigated using molecular dynamic simulations. The mechanism of defect recombination was discussed, as was the evaluation of the recombination radius for Fe and Fe-Cr-Al. The simulation results show that Fe and Fe-Cr-Al have big differences in threshold displacement energies and directional dependencies at low temperatures, but the differences become less pronounced at high temperatures. The cause might be that the replacement collision sequence can effectively separate an interstitial from a vacancy in Fe-Cr-Al and greatly reduce threshold displacement energy at low temperatures. In addition, the energy transfer efficiency of primary knock-on Al atoms is low due to the difference in atomic mass, and the recombination effect for Cr is hindered by the formation of Fe-Cr 〈110〉 dumbbell. However, the effect of solute atoms on threshold displacement energy is insignificant at high temperatures, due to the increase in the recombination radius and a reduction in the effect of the aforementioned factors.