Understanding the intrinsic correlation between helium concentration and the evolution of defects as well as mechanical properties in low-activation steel on an atomic scale is crucial for designing fusion materials with excellent resistance to swelling and embrittlement. This study investigates the effect of helium concentration on single-crystal iron through molecular dynamics simulations, thereby clarifying the mechanisms by which helium concentration affects helium defect evolution, mechanical properties, and plastic deformation behavior of low-activation steel on an atomic scale. Models of body-centered cubic (BCC) iron with different helium concentrations (0.5%—4.5%) are established. Wigner-Seitz cell analysis and cluster clustering methods are employed to track the evolution of Frenkel Pairs (FPs) and cluster defects, revealing the mechanism of helium concentration-induced FPs and cluster formation at 500 ℃. Furthermore, combined with tensile mechanical simulations, the effects of helium behavior on the mechanical properties of single-crystal iron, such as elastic modulus, yield strength, and toughness, are analyzed, and the correlation mechanisms between helium concentration-induced defect evolution, mechanical properties, and plastic deformation behavior are revealed. The results show that when N_He < 3.0%, the number of FPs linearly reaches to a peak and then stabilizes. This is because helium behavior causes a rapid increase in the number of FPs and a large number of interstitial atoms are generated, some of which recombine. The annihilation rate of FPs increases with their number increasing and eventually equals the generation rate, resulting in a stable number of FPs. When N_He ≥ 3.0%, the initial increase and stabilization are the same as those for N_He < 3.0%. However, after the formation of large interstitial clusters, they absorb interstitial atoms and grow, hindering recombination and reducing the annihilation rate of FPs, thus leading to a secondary increase. The large clusters are surrounded by vacancies and no longer hinder FP recombination, and a new balance is achieved, resulting in a secondary stabilization of the FP number. When N_He increases to 3.0%, the elastic modulus, yield strength, and toughness of single-crystal iron decrease by 21%, 88%, and 57%, respectively; beyond this concentration, the mechanical properties no longer decrease. This is because when N_He < 3.0%, as helium concentration increases, helium-induced defects increase, leading to a decrease in toughness and promoting dislocation nucleation, thus reducing the elastic modulus and yield strength. When N_He ≥ 3.0%, dislocations exist in the initial defects, and the number of clusters changes slightly; toughness no longer decreases, and dislocation nucleation is not affected, leading to the stabilization of elastic modulus and yield strength. At N_He = 3.0%, the formation of large clusters hinders the movement of slip systems, changes the orientation of slip planes, weakens the effectiveness of the main slip system, which leads to an increase in small slip bands and causes the plastic deformation mechanism to transform from cross-slip to decomposition into discrete dislocations and point defects once the slip bands intersect with each other. This study reveals the influence patterns and key mechanisms of helium concentration on defect evolution and mechanical properties of single-crystal iron, providing a theoretical basis for designing fusion iron-based materials.