Abstract: Iron-based structural materials exhibit a significant hydrogen-helium synergistic damage effect under 14 MeV fusion neutron irradiation. Currently, the researched irradiation-resistant fusion reactor structural materials mainly have high-density nano-oxide particles or precipitated phases, which mean abundant defects such as grain boundaries and phase interfaces acting as traps. This helps to disperse vacancies, suppress their growth, and thus enhance the material’s resistance to the synergistic damage caused by hydrogen and helium from fusion neutron irradiation. Although grain boundaries, phase boundaries, and dislocations can all capture cavities, the aggregation of cavities at grain boundaries has a more detrimental effect on the mechanical properties of the material than their aggregation at phase boundaries or dislocations. Therefore, studying the capture capabilities of different defect sinks on cavities are of great guiding significance for the development of new materials that are more effective in withstanding fusion neutron irradiation. Helium plays a dominant role in cavity nucleation and growth. This work adopts the method of helium ion injection to obtain the relative strength of various defect sinks in capturing helium by observing the aggregation degree of helium bubbles around different defect sinks. The micro-nano composite 304L stainless steels (MN304-La) mainly have four types of defect sinks, which are nano-precipitate (NP), composite grain boundary (CGB), nanograin boundary (NGB), and dislocations. In order to study the relative capture strength of helium by these defect sinks, three types of MN304-Las with different nanograin sizes but the same microcrystalline grain sizes were irradiated by 190 keV He⁺ ions to a fluence of 1.5×10¹⁶ cm⁻² at room temperature (RT) and 450 ℃, respectively. Samples irradiated at RT were subsequently annealed at 900 ℃. Transmission electron microscope (TEM) and quantitative measurement were used to characterize the distribution of helium bubbles on different defect sinks in three MN304-La samples. The method proposed before was used and it is found that among the four defect sinks, the strongest capture strength for helium in 259-MN304-La is possessed by the NP, the second-strongest capture strength is possessed by CGBs, and the weakest capture strength is possessed by dislocations. There exists a grain environment effect on the capability of NPs to capture helium. As the nanograin size decreases, the capability of NPs to capture helium will be weakened by surrounding NGBs, so the capability of NPs to capture helium becomes the weakest among the four defect sinks. Additionally, increasing the temperature enhances the capability of each defect sink to capture helium, though the relative strength of helium captured by the four defect sinks in each material remains unchanged.