14 MeV fusion neutrons induce significant displacement damage in materials and generate transmutation gases such as hydrogen (H) and helium (He) through (n,p) and (n,α) nuclear reactions. Previous studies have shown that the H-He synergistic effects with displacement defects accelerate degradation in materials. To explore the impact of H-He synergistic effects on cavity nucleation and growth at different temperatures, systematic experiments were performed using the Xiamen Multiple Ion Beam In situ Analysis TEM Facility. In this study, pure nickel samples were irradiated to a damage level of approximately 12 displacements per atom (dpa) at temperatures ranging from 300 °C to 600 °C with varying concentrations of implanted gas atoms. The irradiation configurations and corresponding H–He injection ratios were as follows: single beam (Ni, 0–0 appm dpa−1), dual beam (Ni + He, 0–10 appm dpa−1), dual beam (Ni + H, 50–0 appm dpa−1), and triple beam (Ni + He + H, 40–10 appm dpa−1).The results revealed a distinct transition from nucleation-dominated to growth-dominated cavity evolution with increasing temperature. At lower temperatures, cavity formation is primarily governed by nucleation processes, whereas at 600 °C, pronounced cavity growth becomes dominant after initial nucleation. Helium plays a crucial role in stabilizing vacancy clusters and promoting cavity nucleation, while hydrogen exhibits a considerably weaker influence. Nevertheless, under triple ion beam irradiation conditions, hydrogen promotes the nucleation-driven evolution at 450 °C but contributes more to growth-driven evolution at 600 °C by accumulating around larger cavities.