The synergistic effect associated with displacement damage, hydrogen and helium in the China Low Activation Martensitic (CLAM) steel has been investigated using the triple ion beam irradiation. Triple ion beams, an iron beam of 109 MeV degraded by a tantalum foil of 7.45 μm thick, the 100 keV hydrogen and 200 keV helium, were injected into the CLAM steel samples simultaneously or sequentially. The radiation damage examinations were carried out by the slow positron Doppler broadening technique. The measured S parameters indicate that the radiation damage is different for different irradiation procedures with the same dpa and concentrations of H and He. The sample suffered most severe damage in the simultaneous triple beam irradiation. The present experimental results support the molecular dynamics simulation result that hydrogens facilitate the He-bubble nucleation and growth. © 2016 Trans Tech Publications, Switzerland.