Abstract: Ferrite/martensite steel is a candidate structural material for nuclear reactor. Cr clusters generated after irradiation will change the mechanical properties of the material, leading to hardening and embrittlement. It is helpful to understand its radiation hardening mechanism to study it deeply, which is of great significance for improving the service performance of ferrite/martensite steel. Nanoindentation simulation can combine the evolution of micro-defects and the changes of macro-mechanical properties to explore the physical mechanisms of the changes of mechanical properties of materials, and is an important tool to explore the current topic selection. In order to analyze the different effects of solid solution Cr element and Cr precipitation on the matrix, molecular dynamics was used to simulate the nanoindentation of pure Fe, Fe-12Cr alloy and Fe with Cr clusters in this paper. At the same time, three crystal orientations 100, 110 and 111 were selected to explore the effects of crystal orientation on the mechanical properties of the crystals, and the following results are finally obtained. The results of nanoindentation hardness show that the hardness of 111 crystal orientation is the highest and that of 100 crystal orientation is the lowest, which reflects the anisotropy of crystals, since the difficulty of activating slip systems and the number of activated slip systems are different under different crystal orientations, the hardness in different crystal orientations is also different. And compared with Fe-12Cr alloy, the Fe matrix with Cr clusters has obviously hardened, which shows that the precipitation of Cr will lead to the hardening of Fe compared with the solid solution of Cr element. By analyzing the dislocations, it is found that the dislocation length in Fe matrix containing Cr clusters decreases obviously, while the nucleation time of dislocations and the depth of indenter when dislocations nucleate slightly increase, indicating that the addition of Cr clusters slightly slows down the nucleation of dislocations and significantly hinders the expansion of dislocations in the matrix. Observing the interaction between dislocations and Cr clusters in the process of nanoindentation simulation, it is found that dislocations contact with Cr clusters during the movement, indicating that dislocations need to pass through Cr clusters to move forward, which lead to the decrease of dislocation length and plastic volumes in Fe matrix with Cr clusters. At the same time, the resistance of Cr clusters to dislocation motion is the fundamental reason for matrix hardening.