Self-trapping may play an important role in helium radiation damage of W material in fusion reactors. But the effect of rhenium (Re) and tantalum (Ta) on the self-trapping process in the defect-free tungsten is not clear at present, which has been investigated by the first-principles calculation in this work. The results suggest that He can be trapped by other He atoms without prior defects in the presence of Re/Ta substitutional atom. The substitutional Re atom acts as a trap for the interstitial He clusters and enhances their nucleation, while the Ta atom repulses the large He clusters (n ⩾ 5) and weakens the cluster bonding, thus the growth of He clusters in the vicinity of Ta is not energetically favorable. Moreover, the critical content of He atoms that may trigger the trap mutation decreases when Re exists but increases when Ta exists. These results suggest Re can facilitate the growth of He clusters by self-trapping while Ta has the opposite effects.